Medical Hypotheses Medico/ Hyp.dms (1992) 3&Z%-239 rDbmgnmGmpUKLtd1992
L-Arginine: A Therapeutic Option for AIDS/HIV Infection? M. N. WRIGHTHAM, A. J. CANN* and H. F. SEWELL Department of Immunology, University Hospital, Queen’s Medical Centre, Nottingham, NG7 2UH, UK; ‘Department of Microbiology, University of Leicestec LEl 9HN, UK
Abstract-Numerous studies implicate cellular immunological effector systems in the partial containment of virus replication during the early stages of HIV infection. lmmunostimulatory therapeutic regimes designed to enhance virus clearance are therefore theoretically attractive, but are accompanied by the risk of concomitant activation of HIV replication. Supra-normal levels of l_-arginine have been shown to induce broad immune stimulation in vitro and in vivo, but do not increase HIV gene expression in vitro. These observations, together with the lack of toxicity of this agent, suggest a novel therapeutic approach to HIV disease.
Introduction Most of the existing models of the pathogenesis of acquired immune deficiency syndrome (AIDS) regard the eventual attrition of the immune system as a direct or indirect consequence of human immunodeficiency virus (HIV) replication in cell populations critical to the immune response, leading to regulatory disturbances, autoimmune phenomena or cytopathic effects (1, 2, 3). This can be interpreted as a two stage process in which the host immune system initially contains virus propagation but eventually succumbs to progressive destruction resulting in clinical immune suppression. Therapeutic strategies based on stimulation of anti-viral immunological effector systems are therefore attractive in principle, particularly in the early stages of the disease course, but such approaches may carry the attendant risk of simultaneously increasing turnover within the latent HIV pool, a double-edged sword effect resulting from Date received 8 January 1992 Date accepted 10 February 1992
the subversion of normal cellular activation signals by the virus in order to drive its own replication (4). Dietary supplementation with supra-optimal doses of L-arginine in vivo has been shown to markedly enhance the activity of various cellular effector systems with potential antiviral action but, crucially, may not deliver signals which induce transcription from the viral long terminal repeat (LTR). L-arginine also has a proven safety record with negligible toxicity in humans. These observations suggest a potential therapeutic approach to HIV infection based on arginine supplementation alone or in combination with established nucleoside analogues which have known antiHIV activity. Cellular responses to HIV in vivo
Recent data indicate that symptomatic primary HIV infection is associated with high titres of serum and
236
L-ARC&-WE: A THERAPEUK
237
OPTION FOR AIDS/HIV INFECI’ION?
cell-associated virus which fall precipitously, implying the action of host immunological effector mechanisms (5, 6). The ensuing asymptomatic phase is initially character&d by vigorous antiviral humoral and cellular immune responses, of which at least some components are implicated in the apparent control of virus propagation (7, 8). Natural killer (NK) cells from seronegative donors are known to have lytic activity against HIV-infected T lymphoblastoid (9) and monoblastoid (10) cell lines. Qualitatively and quantitatively similar cytotoxic activity is also present in peripheral blood mononuclear cells from HIV seropositive donors in the early stages of the disease course (10). Antibody-dependent cellular cytotoxicity (ADCC) mechanisms can also potentially contribute to the destruction of infected cells, and evidence now suggests the presence of circulating killer cell effecters ‘pre-armed’ in vivo with antibody to the viral envelope in infected individuals (11, 12). Classical MHC class I-restricted cytotoxic T cells (CTL) are also likely to contribute to the clearance of infected cells, as remarkably high levels of CTL directed against various HIV structural or regulatory proteins are apparent in asymptomatic infected humans (13, 14, 15). The CD8+ cell fraction from seropositive donors is additionally able to suppress virus replication in autologous CD4+ cells by a non-cytolytic mechanism (16), which appears to be cytokine-dependent (17). All of these cellular anti-viral activities are functionally intact in the early asymptomatic phase of infection, but decline prior to the onset of AIDS (18-22).
Immunostimulation by supplemental L-arginine in vivo The basic amino acid L-arginine is well characterised at the biochemical and physiological levels, and initially attracted experimental attention as a result of reported beneficial effects as a dietary supplement in various animal tumour models (reviewed in (23)). These findings have more recently been extended to show that arginine supplementation also results in stimulation of several cellular immunological effector systems in both mice and humans. Mice receiving supplemental arginine show significant increases in the cytolytic potential of activated NK cells and macrophages in standard assay systems (24). together with enhanced allospecific CTL activity (25). Parallel increases in T cell mitogen response in vitro were also observed in these mice. The mechanism underlying these effects is unknown, although the enhancement of NK and CTL activity at least is reproduced by
direct exposure of cells to pharmacologically relevant doses of arginine in vitro, suggesting that this simpler system contains all necessary components for the stimulatory effect and that the known endocrine secretagogue activity of arginine (23) is unlikely to be involved in these processes. Arginine is a precursor for the production of nitric oxide by cells of macrophage lineage, and recent studies suggest that this molecule may have various inter- and intracellular signalling roles (26). Arginine is also a precursor for polyamine biosynthesis, suggesting a second pathway by which some or all of the observed effects might be achieved, and the possibility of other entirely distinct mechanisms cannot be excluded. If the mode of action of arginine in vitro is assumed to parallel that occurring in vivo, direct assay systems under cell culture conditions could greatly facilitate the necessary mechanistic studies. Supplemental arginine is also known to enhance T cell responsiveness to the mitogens concanavalin A and phytohaemagglutinin (Con A/PHA) in humans (27, 28). We have recently extended these findings to show that in human volunteers, oral arginine induces increases in circulating cells carrying the NKassociated marker CD56, together with pronounced increases in spontaneous NK cytolytic activity (29). These changes are accompanied by enhancement of the capacity of peripheral blood mononuclear cells to generate lymphokine-activated killer (LAK) activity on subsequent incubation in vitro with interleukin-2, a finding which further indicates a stimulatory effect within the large granular lymphocyte (LGL) compartment. NK and ADCC activities appear to utilise a common lytic mechanism once triggering has occurred (30), and indeed are properties of largely overlapping cell populations (31) suggesting that the potentiating effects of arginine should also extend to the latter effector system. Extrapolation from the murine system strongly suggests that arginine may have similar effects on classical CR-mediated lysis in humans. In common with the mouse, human NK enhancement is reproduced on in vitro incubation in culture medium with supra-normal arginine levels (29). and the likely effects of this agent on other cytolytic effecters may be initially testable in such a system. Therapeutic potential of L-arginine Several considerations suggest that therapeutic strategies based on L-arginine should be more beneficial if introduced early in the disease course, not least because of the higher intrinsic activity of the relevant host effector systems during the asymptomatic period (18-22). Various models suggest that HIV patho-
238 genecity is associated with viral evolution in vivo to variants with altered replicative and cytopathic properties (2.32) or with progressive virus diversification beyond the capacity of the immune system to contain the infection (33), considerations which also imply a need for early intervention. Any immunostimulatory approach to tmatment of HIV disease must take account of the concomitant possibility of triggering increased replication within the latent virus pool, although the likely outcome of this scenario is somewhat unclear. If a limited effect of this kind were to occur in the context of increased anti-HIV immune responsiveness, the cumulative effects could well be beneficial, with the ‘hidden agenda’ of the virus being partially exposed to enhanced immunological clearance; such a situation is of course more likely to prevail in the earlier stages of the natural history of HIV infection. The various physiological signals currently known to induce transcription from the viral long terminal repeat (LTK) are predominantly receptor-mediated stimuli such as antigen receptor ligation (34) and cytokines (35, 36), which form part of the normal immunoregulatory network. The effect of arginine on the immune system may well be distinct from these, and could even constitute a non-specific secondary stimulus which does not ordinarily play any physiolgical regulatory role. On this basis it might therefore be anticipated that supplemental arginine could separate immunostimulation from latent virus activation, and experiments in our laboratories suggest that this is indeed the case, as arginine does not activate the HIV LTR in assays based on transient expression of an associated bacterial chloramphenicol acetyltransferase (CAT) reporter gene in established T cell lines, under conditions which induce marked NK cell stimulation in vitro. Similar results are also seen in primary PHAstimulated peripheral mononuclear cell preparations, a system which may bear closely upon the situation in vivo. The theoretical possibility still exists that L-arginine treatment, while not directly stimulating the viral LTR, may induce some degree of virus replication in vivo as a secondary consequence of increases in cytokine levels which might be associated with its diverse modulating effects. This issue could be investigated in a small animal model either by experiments analogous to the above using transgenic mice carrying integrated copies of LTR-CAT constructs (37) or, more conclusively, using human-PBL-SCID mice infected with HIV (38). In a clinical setting, reassurance against virus release from the latently infected cell pool could be obtained, if necessary, by a combi-
MEDICAL HYPOTHESES
natorial therapeutic regimen incorporating zidovudine (AZT) or other known anti-retroviral nucleoside analogues, to limit the extent of secondary reinfection of bystander cells. Conclusion The breadth of action of supplemental arginine indicates that it might substitute for multiple recombinant cytokines (which have been proposed as a modality for treating HIV infection) without the associated toxicity; human volunteers experience no obvious side-effects after arginine doses of at least 3Og/day (29). The associated lack of expense, stability and ease of administration of this agent would also be attractive, particularly in the developing world where serious economic, organisational and logistical difficulties hamper the response to the AIDS pandemic, We suggest that the immunomodulatory action of Larginine, alone or in combination with anti-retroviral nucleoside analogues, may be of therapeutic value in HIV infection and merits further investigation. References 1. Rosenberg Z F, Fauci A S. The immunopathogenesis of HIV infection. Adv Immunol 47:377-431, 1989. 2. Miedema F, Tersmette M, Van Lier R A W. AIDS pathogenesis: a dynamic interaction between HIV and the immune system. lmmunol Today 11: 293-297, 1990. 3. Habeshaw J A. Dalgleish A G, Bountiff L, NewelI A L, Wilks D, Walker L C, Manta F. AIDS pathogenesis: HIV envelope and its interaction with cell proteins. Immunol Today 11: 418425. 1990. 4. Greene W C, Bohnlein E, Ballard D W. HIV-l, HTLV-1 and normal T cell growth: transcriptional strategies and surprises. Immunol Today 10: 272-278, 1989. 5. Clark S J, Saag M S. Decker W D, Campbell-Hill S, Roberson J L, Veldkamp P J, Kappes J C, Hahn B H, Shaw G M. High titres of cytopathic virus in plasma of patients with symptomatic primary HIV-l infectian. New Engl J Med 324: 954-960, 1991. 6. Daar E S, Moudgil T, Meyer R D, Ho D D. Transient high levels of viremiaut patients with primary human immunodeficiencv virus infection. New Enel J Med 324: 961-%4. 1991. 7. Bolognesi D P. HIV antibodies-and vaccine design. AIDS 3 (suppl 1): SlllSlI8, 1989. 8. Mills K H G, Nixon D F, McMichael A J. T cell strategies in AIDS vaccines: MHC-restricted T cell responses to HIV proteins. AIDS 3(suppl 1) SlOl-SllO, 1989. 9. Bandyopadhyay S, Ziegner U, Campbell D E, Miller D S, Hoxie J A. Starr S E. Natural killer cell-mediated lvsis of T cell lines chronically infected with HIV-I. Clin Exp immunol 79: 430-43s. 1990. 10. Rappocciolo G, Toso J F, Torpey D J, Gupta P, Rinaldo C R. Association of alpha-interferon production with natural killer cell lysis of U937 cells infected with human immunodeticiency virus. J Clin Microbial 27: 41-49, 1989. 11. Tyler D S, Nastala C L, Stanley S D, Matthews T J, Lyerly H K, Bolognesi D P, Weinhold K J. Gpl20-specific cellular
239
L-ARGININE: A THERAPEUTIC OPIlON FOR AIDS/HIV INFECIION?
12.
13.
14.
15
16
17.
18.
19.
20.
21.
22.
23. 24.
cytotoxicity in HIV-1 seropositive individuals: evidence for circulating CD16+ effector cells armed in viva with cytophilic antibody. J Immunol 142: 1177-1182. 1989. Tanneau F, McChesney M, Lopez 0. Sansonetti P, Montagnier L, Riviere Y. Primary cytotoxicity against the envelope glycoprotein of human immunodeficiency virus-I: evidence for antibody-dependent cellular cytotoxicity in viva. J Inf Dis 162: 837-843, 1990. Walker B D, Chakrabarti S. Moss B. Paradis T J, Flynn 7: Dumo A G, Blumberg R S, Kaplan J C, Hirsch M S, Schoo ley R T. HIV-specific cytotoxic T lymphocytes in seropositive individuals. Nature 328: 345-348, 1987. Nixon D F, Townsend A R M. Elvin J G. Rizza C R. GaBwey J, McMichael A J. HIV-I gag-specific cytotoxic T lymphocytes defined with recombinant vaccinia virus and synthetic peptides. Nature 336: 484487, 1988. Koenig S, Fuerst T R, Wood L V, Woods R M, Suzich J A, Jones G M, De La Cmz V F, Davey R T, Venkatesan S, Moss B, Biddison W E, Fauci A S. Mapping the fine specificity of a cytolytic T cell response to HIV-I nef protein, J Immunol 145: 127-135, 1990. Wiviott L D, Walker C M, Levy J A. CD8+ lymphocytes suppress HIV production by autologous CD4+ cells without eliminating the infected cells from culture. Cell Immunol 128: 628634, 1990. Walker C M, Levy J A. A diffusible lymphokine produced by CD8+ T lymphocytes suppresses HIV replication. Immunology 66: 628-630, 1989. Brenner B G, Dascal A, Margolese R G, Wait&erg M A. Natural killer cell function in patients with acquired immunodeficiency syndrome and related diseases. J Leukocyte Biol 46: 75-83, 1989. Weinhold K J, Lyerly H K, Matthews T J, Tyler D S, Aheame P M, Stine K C. Langlois A J. Durack D T, Bolognesi D P. Cellular anti-gp 120 cytolytic reactivities in HIV-I seropositive individua%. La&t i: 902-905, 1988. Tvler D S. Stanlev S D. Nastala C A. Austrin A A. Bartlett J-A, Stine’ K C, Lyerly ‘H K, Bologndsi D P, Weinhold K J. Alterations in antibody-dependent cellular cytotoxicity during the course of HIV-I infection. J lmmunol 144: 3375-3384, 1990. Hoffenbach A, Langlade-Demoyen P, Dadaglio G, Vilmer E, Michel F, Mayaud C. Autran B, Plats F. Unusually high frequencies of HIV-specific cytotoxic T lymphocytes in humans. J Immunol 142: 452-462, 1989. Mackewicz C E, Ortega H W, Levy J A. CDS+ cell anti-HIV activity correlates with the clinical state of the infected individual. J Clin Invest 87: 14621466, 1991. Barbel A. Arginine: Biochemistry, physiology and therapeutic implications. J Parenteral Enteral Nutr 10: 227-238, 1986. Reynolds J V, Daly J M, Shou J, Sigal R, Ziegler M M, Naji A. Immunologic effects of arginine supplementation in
25.
26.
27.
28.
29.
30.
31.
32.
33.
34.
35.
36.
37.
38.
tumour-bearing and non-tumour bearing hosts. Ann Surg 211: 202-210, 1990. Reynolds J V, Daly J M. Zhang S, Evantash E, Shou J. Sigal R. Ziegler M M. Immunomodulatory mechanisms of arginine. Surgery 104: 142-151, 1988. Moncada S, Palmer R M J. Higgs E A. Biosynthesis of mtric oxide from L-arginine; a pathway for the regulation of cell function and communication. Biochem Phamracol 38: 1709-1715, 1989. Barbul A, Sisto D A, Wasserkrug B A. Efron G. Arginine stimuates lymphocyte immune response in healthy human beings. Surgery 90: 244-251, 1981. Daly J M. Reynolds J, Tbom A, Kinsley L, Diet&k-Gallagher M, Shou J. Ruggieri B. Immune and metabolic effects of arginine in the surgical patient. Ann Surg 208: 512-521, 1988. Park K G M. Hayes P D, Garlick P J, SeweU H F, Eremin 0. Stimulation of lymphocyte natural cytotoxicity by Larginine in man. lancet 337: 645-646, 1991. Abrams S I, Brahmi Z. Target cell directed NK inactivation: concomitant loss of NK and antibody-dependent cellular cytotoxicity activities. J lmmunol 140: 2090-2095, 1988. De Landazuri M 0, Silva A, Alvarez J. Herberman R B. Evidence that natural cytotoxicity and antibody-dependent cellular cytotoxicity are mediated in humans by the same effector cell populations. J Immunol 123: 252-258, 1979. Fenyo E M, Albert J, Asjo B. Replicative capacity, cytopathic effect and cell tropism of HIV. AIDS 3 (suppl 1) S5S12. 1989. Nowak M A, May R M, Anderson R M. The evolutionary dynamics of HIV-I quasispecies and the development of immunodeficiency disease. AIDS 4: 1095-1103, 1990. Tong-Starksen S E, Luciw P A, Peterlin B M. SignaUing through T lymphocyte surface proteins, TCR/CD3 and CD28. activates the HIV-I long terminal repeat. J Immunol 142: 702-707, 1989. Osbom L, Kunkel S. Nabel G J. Tumor necrosis factor and interleukin 1 stimulate the human immunodeficiency virus erhancer by activation of the nuclear factor KB. Proc Nat1 Acad Sci USA 86: 2336-2340. 1989. Duh E J, Maury W J, Folks T M. Fauci A S. Rabson A B. Tumor necrosis factor alpha activates human immunodeficiency virus type 1 through induction of nuclear factor binding the NF-KB sites in the long terminal repeat. Proc Natl Acad Sci USA. 86: 5974-5978, 1989. Leonard J, Khillan J S, Gendelman H E, Adachi A, Lorenzo S, Westphal II, Martin M A, Meltzer M S. The human immunodeficiency virus long terminal repeat is preferentially expressed in Langerhans cells in transgenic mice. AIDS Res Hum Retroviruses 5: 421-430, 1989. Mosier D E, Guilizia R J, Baird S M, Wilson D B, Spector D H, Spector S A. Human immunodeliciency virus infection of human PBL-SCID mice. Science 251: 791-794, 1991.
L-Arginine: A Therapeutic Option for AIDS/HIV Infection? M. N. WRIGHTHAM, A. J. CANN* and H. F. SEWELL Department of Immunology, University Hospital, Queen’s Medical Centre, Nottingham, NG7 2UH, UK; ‘Department of Microbiology, University of Leicestec LEl 9HN, UK
Abstract-Numerous studies implicate cellular immunological effector systems in the partial containment of virus replication during the early stages of HIV infection. lmmunostimulatory therapeutic regimes designed to enhance virus clearance are therefore theoretically attractive, but are accompanied by the risk of concomitant activation of HIV replication. Supra-normal levels of l_-arginine have been shown to induce broad immune stimulation in vitro and in vivo, but do not increase HIV gene expression in vitro. These observations, together with the lack of toxicity of this agent, suggest a novel therapeutic approach to HIV disease.
Introduction Most of the existing models of the pathogenesis of acquired immune deficiency syndrome (AIDS) regard the eventual attrition of the immune system as a direct or indirect consequence of human immunodeficiency virus (HIV) replication in cell populations critical to the immune response, leading to regulatory disturbances, autoimmune phenomena or cytopathic effects (1, 2, 3). This can be interpreted as a two stage process in which the host immune system initially contains virus propagation but eventually succumbs to progressive destruction resulting in clinical immune suppression. Therapeutic strategies based on stimulation of anti-viral immunological effector systems are therefore attractive in principle, particularly in the early stages of the disease course, but such approaches may carry the attendant risk of simultaneously increasing turnover within the latent HIV pool, a double-edged sword effect resulting from Date received 8 January 1992 Date accepted 10 February 1992
the subversion of normal cellular activation signals by the virus in order to drive its own replication (4). Dietary supplementation with supra-optimal doses of L-arginine in vivo has been shown to markedly enhance the activity of various cellular effector systems with potential antiviral action but, crucially, may not deliver signals which induce transcription from the viral long terminal repeat (LTR). L-arginine also has a proven safety record with negligible toxicity in humans. These observations suggest a potential therapeutic approach to HIV infection based on arginine supplementation alone or in combination with established nucleoside analogues which have known antiHIV activity. Cellular responses to HIV in vivo
Recent data indicate that symptomatic primary HIV infection is associated with high titres of serum and
236
L-ARC&-WE: A THERAPEUK
237
OPTION FOR AIDS/HIV INFECI’ION?
cell-associated virus which fall precipitously, implying the action of host immunological effector mechanisms (5, 6). The ensuing asymptomatic phase is initially character&d by vigorous antiviral humoral and cellular immune responses, of which at least some components are implicated in the apparent control of virus propagation (7, 8). Natural killer (NK) cells from seronegative donors are known to have lytic activity against HIV-infected T lymphoblastoid (9) and monoblastoid (10) cell lines. Qualitatively and quantitatively similar cytotoxic activity is also present in peripheral blood mononuclear cells from HIV seropositive donors in the early stages of the disease course (10). Antibody-dependent cellular cytotoxicity (ADCC) mechanisms can also potentially contribute to the destruction of infected cells, and evidence now suggests the presence of circulating killer cell effecters ‘pre-armed’ in vivo with antibody to the viral envelope in infected individuals (11, 12). Classical MHC class I-restricted cytotoxic T cells (CTL) are also likely to contribute to the clearance of infected cells, as remarkably high levels of CTL directed against various HIV structural or regulatory proteins are apparent in asymptomatic infected humans (13, 14, 15). The CD8+ cell fraction from seropositive donors is additionally able to suppress virus replication in autologous CD4+ cells by a non-cytolytic mechanism (16), which appears to be cytokine-dependent (17). All of these cellular anti-viral activities are functionally intact in the early asymptomatic phase of infection, but decline prior to the onset of AIDS (18-22).
Immunostimulation by supplemental L-arginine in vivo The basic amino acid L-arginine is well characterised at the biochemical and physiological levels, and initially attracted experimental attention as a result of reported beneficial effects as a dietary supplement in various animal tumour models (reviewed in (23)). These findings have more recently been extended to show that arginine supplementation also results in stimulation of several cellular immunological effector systems in both mice and humans. Mice receiving supplemental arginine show significant increases in the cytolytic potential of activated NK cells and macrophages in standard assay systems (24). together with enhanced allospecific CTL activity (25). Parallel increases in T cell mitogen response in vitro were also observed in these mice. The mechanism underlying these effects is unknown, although the enhancement of NK and CTL activity at least is reproduced by
direct exposure of cells to pharmacologically relevant doses of arginine in vitro, suggesting that this simpler system contains all necessary components for the stimulatory effect and that the known endocrine secretagogue activity of arginine (23) is unlikely to be involved in these processes. Arginine is a precursor for the production of nitric oxide by cells of macrophage lineage, and recent studies suggest that this molecule may have various inter- and intracellular signalling roles (26). Arginine is also a precursor for polyamine biosynthesis, suggesting a second pathway by which some or all of the observed effects might be achieved, and the possibility of other entirely distinct mechanisms cannot be excluded. If the mode of action of arginine in vitro is assumed to parallel that occurring in vivo, direct assay systems under cell culture conditions could greatly facilitate the necessary mechanistic studies. Supplemental arginine is also known to enhance T cell responsiveness to the mitogens concanavalin A and phytohaemagglutinin (Con A/PHA) in humans (27, 28). We have recently extended these findings to show that in human volunteers, oral arginine induces increases in circulating cells carrying the NKassociated marker CD56, together with pronounced increases in spontaneous NK cytolytic activity (29). These changes are accompanied by enhancement of the capacity of peripheral blood mononuclear cells to generate lymphokine-activated killer (LAK) activity on subsequent incubation in vitro with interleukin-2, a finding which further indicates a stimulatory effect within the large granular lymphocyte (LGL) compartment. NK and ADCC activities appear to utilise a common lytic mechanism once triggering has occurred (30), and indeed are properties of largely overlapping cell populations (31) suggesting that the potentiating effects of arginine should also extend to the latter effector system. Extrapolation from the murine system strongly suggests that arginine may have similar effects on classical CR-mediated lysis in humans. In common with the mouse, human NK enhancement is reproduced on in vitro incubation in culture medium with supra-normal arginine levels (29). and the likely effects of this agent on other cytolytic effecters may be initially testable in such a system. Therapeutic potential of L-arginine Several considerations suggest that therapeutic strategies based on L-arginine should be more beneficial if introduced early in the disease course, not least because of the higher intrinsic activity of the relevant host effector systems during the asymptomatic period (18-22). Various models suggest that HIV patho-
238 genecity is associated with viral evolution in vivo to variants with altered replicative and cytopathic properties (2.32) or with progressive virus diversification beyond the capacity of the immune system to contain the infection (33), considerations which also imply a need for early intervention. Any immunostimulatory approach to tmatment of HIV disease must take account of the concomitant possibility of triggering increased replication within the latent virus pool, although the likely outcome of this scenario is somewhat unclear. If a limited effect of this kind were to occur in the context of increased anti-HIV immune responsiveness, the cumulative effects could well be beneficial, with the ‘hidden agenda’ of the virus being partially exposed to enhanced immunological clearance; such a situation is of course more likely to prevail in the earlier stages of the natural history of HIV infection. The various physiological signals currently known to induce transcription from the viral long terminal repeat (LTK) are predominantly receptor-mediated stimuli such as antigen receptor ligation (34) and cytokines (35, 36), which form part of the normal immunoregulatory network. The effect of arginine on the immune system may well be distinct from these, and could even constitute a non-specific secondary stimulus which does not ordinarily play any physiolgical regulatory role. On this basis it might therefore be anticipated that supplemental arginine could separate immunostimulation from latent virus activation, and experiments in our laboratories suggest that this is indeed the case, as arginine does not activate the HIV LTR in assays based on transient expression of an associated bacterial chloramphenicol acetyltransferase (CAT) reporter gene in established T cell lines, under conditions which induce marked NK cell stimulation in vitro. Similar results are also seen in primary PHAstimulated peripheral mononuclear cell preparations, a system which may bear closely upon the situation in vivo. The theoretical possibility still exists that L-arginine treatment, while not directly stimulating the viral LTR, may induce some degree of virus replication in vivo as a secondary consequence of increases in cytokine levels which might be associated with its diverse modulating effects. This issue could be investigated in a small animal model either by experiments analogous to the above using transgenic mice carrying integrated copies of LTR-CAT constructs (37) or, more conclusively, using human-PBL-SCID mice infected with HIV (38). In a clinical setting, reassurance against virus release from the latently infected cell pool could be obtained, if necessary, by a combi-
MEDICAL HYPOTHESES
natorial therapeutic regimen incorporating zidovudine (AZT) or other known anti-retroviral nucleoside analogues, to limit the extent of secondary reinfection of bystander cells. Conclusion The breadth of action of supplemental arginine indicates that it might substitute for multiple recombinant cytokines (which have been proposed as a modality for treating HIV infection) without the associated toxicity; human volunteers experience no obvious side-effects after arginine doses of at least 3Og/day (29). The associated lack of expense, stability and ease of administration of this agent would also be attractive, particularly in the developing world where serious economic, organisational and logistical difficulties hamper the response to the AIDS pandemic, We suggest that the immunomodulatory action of Larginine, alone or in combination with anti-retroviral nucleoside analogues, may be of therapeutic value in HIV infection and merits further investigation. References 1. Rosenberg Z F, Fauci A S. The immunopathogenesis of HIV infection. Adv Immunol 47:377-431, 1989. 2. Miedema F, Tersmette M, Van Lier R A W. AIDS pathogenesis: a dynamic interaction between HIV and the immune system. lmmunol Today 11: 293-297, 1990. 3. Habeshaw J A. Dalgleish A G, Bountiff L, NewelI A L, Wilks D, Walker L C, Manta F. AIDS pathogenesis: HIV envelope and its interaction with cell proteins. Immunol Today 11: 418425. 1990. 4. Greene W C, Bohnlein E, Ballard D W. HIV-l, HTLV-1 and normal T cell growth: transcriptional strategies and surprises. Immunol Today 10: 272-278, 1989. 5. Clark S J, Saag M S. Decker W D, Campbell-Hill S, Roberson J L, Veldkamp P J, Kappes J C, Hahn B H, Shaw G M. High titres of cytopathic virus in plasma of patients with symptomatic primary HIV-l infectian. New Engl J Med 324: 954-960, 1991. 6. Daar E S, Moudgil T, Meyer R D, Ho D D. Transient high levels of viremiaut patients with primary human immunodeficiencv virus infection. New Enel J Med 324: 961-%4. 1991. 7. Bolognesi D P. HIV antibodies-and vaccine design. AIDS 3 (suppl 1): SlllSlI8, 1989. 8. Mills K H G, Nixon D F, McMichael A J. T cell strategies in AIDS vaccines: MHC-restricted T cell responses to HIV proteins. AIDS 3(suppl 1) SlOl-SllO, 1989. 9. Bandyopadhyay S, Ziegner U, Campbell D E, Miller D S, Hoxie J A. Starr S E. Natural killer cell-mediated lvsis of T cell lines chronically infected with HIV-I. Clin Exp immunol 79: 430-43s. 1990. 10. Rappocciolo G, Toso J F, Torpey D J, Gupta P, Rinaldo C R. Association of alpha-interferon production with natural killer cell lysis of U937 cells infected with human immunodeticiency virus. J Clin Microbial 27: 41-49, 1989. 11. Tyler D S, Nastala C L, Stanley S D, Matthews T J, Lyerly H K, Bolognesi D P, Weinhold K J. Gpl20-specific cellular
239
L-ARGININE: A THERAPEUTIC OPIlON FOR AIDS/HIV INFECIION?
12.
13.
14.
15
16
17.
18.
19.
20.
21.
22.
23. 24.
cytotoxicity in HIV-1 seropositive individuals: evidence for circulating CD16+ effector cells armed in viva with cytophilic antibody. J Immunol 142: 1177-1182. 1989. Tanneau F, McChesney M, Lopez 0. Sansonetti P, Montagnier L, Riviere Y. Primary cytotoxicity against the envelope glycoprotein of human immunodeficiency virus-I: evidence for antibody-dependent cellular cytotoxicity in viva. J Inf Dis 162: 837-843, 1990. Walker B D, Chakrabarti S. Moss B. Paradis T J, Flynn 7: Dumo A G, Blumberg R S, Kaplan J C, Hirsch M S, Schoo ley R T. HIV-specific cytotoxic T lymphocytes in seropositive individuals. Nature 328: 345-348, 1987. Nixon D F, Townsend A R M. Elvin J G. Rizza C R. GaBwey J, McMichael A J. HIV-I gag-specific cytotoxic T lymphocytes defined with recombinant vaccinia virus and synthetic peptides. Nature 336: 484487, 1988. Koenig S, Fuerst T R, Wood L V, Woods R M, Suzich J A, Jones G M, De La Cmz V F, Davey R T, Venkatesan S, Moss B, Biddison W E, Fauci A S. Mapping the fine specificity of a cytolytic T cell response to HIV-I nef protein, J Immunol 145: 127-135, 1990. Wiviott L D, Walker C M, Levy J A. CD8+ lymphocytes suppress HIV production by autologous CD4+ cells without eliminating the infected cells from culture. Cell Immunol 128: 628634, 1990. Walker C M, Levy J A. A diffusible lymphokine produced by CD8+ T lymphocytes suppresses HIV replication. Immunology 66: 628-630, 1989. Brenner B G, Dascal A, Margolese R G, Wait&erg M A. Natural killer cell function in patients with acquired immunodeficiency syndrome and related diseases. J Leukocyte Biol 46: 75-83, 1989. Weinhold K J, Lyerly H K, Matthews T J, Tyler D S, Aheame P M, Stine K C. Langlois A J. Durack D T, Bolognesi D P. Cellular anti-gp 120 cytolytic reactivities in HIV-I seropositive individua%. La&t i: 902-905, 1988. Tvler D S. Stanlev S D. Nastala C A. Austrin A A. Bartlett J-A, Stine’ K C, Lyerly ‘H K, Bologndsi D P, Weinhold K J. Alterations in antibody-dependent cellular cytotoxicity during the course of HIV-I infection. J lmmunol 144: 3375-3384, 1990. Hoffenbach A, Langlade-Demoyen P, Dadaglio G, Vilmer E, Michel F, Mayaud C. Autran B, Plats F. Unusually high frequencies of HIV-specific cytotoxic T lymphocytes in humans. J Immunol 142: 452-462, 1989. Mackewicz C E, Ortega H W, Levy J A. CDS+ cell anti-HIV activity correlates with the clinical state of the infected individual. J Clin Invest 87: 14621466, 1991. Barbel A. Arginine: Biochemistry, physiology and therapeutic implications. J Parenteral Enteral Nutr 10: 227-238, 1986. Reynolds J V, Daly J M, Shou J, Sigal R, Ziegler M M, Naji A. Immunologic effects of arginine supplementation in
25.
26.
27.
28.
29.
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32.
33.
34.
35.
36.
37.
38.
tumour-bearing and non-tumour bearing hosts. Ann Surg 211: 202-210, 1990. Reynolds J V, Daly J M. Zhang S, Evantash E, Shou J. Sigal R. Ziegler M M. Immunomodulatory mechanisms of arginine. Surgery 104: 142-151, 1988. Moncada S, Palmer R M J. Higgs E A. Biosynthesis of mtric oxide from L-arginine; a pathway for the regulation of cell function and communication. Biochem Phamracol 38: 1709-1715, 1989. Barbul A, Sisto D A, Wasserkrug B A. Efron G. Arginine stimuates lymphocyte immune response in healthy human beings. Surgery 90: 244-251, 1981. Daly J M. Reynolds J, Tbom A, Kinsley L, Diet&k-Gallagher M, Shou J. Ruggieri B. Immune and metabolic effects of arginine in the surgical patient. Ann Surg 208: 512-521, 1988. Park K G M. Hayes P D, Garlick P J, SeweU H F, Eremin 0. Stimulation of lymphocyte natural cytotoxicity by Larginine in man. lancet 337: 645-646, 1991. Abrams S I, Brahmi Z. Target cell directed NK inactivation: concomitant loss of NK and antibody-dependent cellular cytotoxicity activities. J lmmunol 140: 2090-2095, 1988. De Landazuri M 0, Silva A, Alvarez J. Herberman R B. Evidence that natural cytotoxicity and antibody-dependent cellular cytotoxicity are mediated in humans by the same effector cell populations. J Immunol 123: 252-258, 1979. Fenyo E M, Albert J, Asjo B. Replicative capacity, cytopathic effect and cell tropism of HIV. AIDS 3 (suppl 1) S5S12. 1989. Nowak M A, May R M, Anderson R M. The evolutionary dynamics of HIV-I quasispecies and the development of immunodeficiency disease. AIDS 4: 1095-1103, 1990. Tong-Starksen S E, Luciw P A, Peterlin B M. SignaUing through T lymphocyte surface proteins, TCR/CD3 and CD28. activates the HIV-I long terminal repeat. J Immunol 142: 702-707, 1989. Osbom L, Kunkel S. Nabel G J. Tumor necrosis factor and interleukin 1 stimulate the human immunodeficiency virus erhancer by activation of the nuclear factor KB. Proc Nat1 Acad Sci USA 86: 2336-2340. 1989. Duh E J, Maury W J, Folks T M. Fauci A S. Rabson A B. Tumor necrosis factor alpha activates human immunodeficiency virus type 1 through induction of nuclear factor binding the NF-KB sites in the long terminal repeat. Proc Natl Acad Sci USA. 86: 5974-5978, 1989. Leonard J, Khillan J S, Gendelman H E, Adachi A, Lorenzo S, Westphal II, Martin M A, Meltzer M S. The human immunodeficiency virus long terminal repeat is preferentially expressed in Langerhans cells in transgenic mice. AIDS Res Hum Retroviruses 5: 421-430, 1989. Mosier D E, Guilizia R J, Baird S M, Wilson D B, Spector D H, Spector S A. Human immunodeliciency virus infection of human PBL-SCID mice. Science 251: 791-794, 1991.
