Clin. exp. Immunol. (1979) 37, 558-561.
MICC cytotoxic effector function of human T lymphocyte subpopulations bearing Fc-receptors for IgG and IgM L. G. LUM, A. V. MUCHMORE, JEAN M. DECKER & R. M. BLAESE Metabolism Branch, DCBD, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205, USA
(Acceptedfor publication 20 March 1979) SUMMARY
Purified subpopulations of human T lymphocytes bearing Fc-IgG and Fc-IgM receptors were studied for their ability to mediate mitogen (PHA) induced cellular cytotoxicity (MICC) to chicken erythrocyte (CRBC) and DBA/Mastocytoma P815Y tumour cell targets. There were marked differences in the ability of the Fc-IgG receptor-bearing T cell (T y) and the Fc- IgM (Tp) to mediating MICC to CRBC and P815Y target cells. Ty cells were very efficient killers of CRBC and Tji cells had no cytotoxic activity to CRBC. On the other hand, both the Ty and Tlt subpopulations were able to mediate MICC to P815Y tumour cell targets. INTRODUCTION Recently, there has been a great deal of interest in human T lymphocyte subpopulations bearing Fc-IgG and Fc-IgM receptors. Several groups have described Fc-IgG and Fc-IgM receptors on human T cell populations (Ferrarini et al., 1975; Ferrarini et al., 1976; Moretta et al., 1976; Gmelig-Meyling, Van der Ham & Ballieux, 1976). T cells bearing Fc-IgG receptors (T'y) have been demonstrated to be suppressor cells and T cells bearing Fc-IgM receptors (Tj) have been demonstrated to be helper cells in a B cell proliferation assay using pokeweed mitogen (Moretta et al., 1977). More recently, Hayward et al. (1978) have demonstrated that Tji cells exposed to concanavalin A could suppress B cell proliferation. However, Ty cells have also been shown to undergo a transition from a Fc-IgG receptor-bearing cell to a Fc-IgM receptor-bearing cell after IgG immune complex interaction (Pichler, Lum & Broder, 1978). This raises the question of whether or not the Fc-receptors expressed on the various subpopulations of T cells correlate with their functions, especially when their Fc-receptors are being modulated by IgG immune complex interaction. In order to clarify further the functional differences between these two human T cell subsets, we prepared T cell subpopulations enriched for Ty and Tj cells and studied their ability to mediate mitogeninduced cellular cytotoxicity to both chicken erythrocyte (CRBC) and DBA/Mastocytoma P815Y tumour cell targets. MATERIALS AND METHODS Purification of human T cells. Human peripheral blood mononuclear cells were obtained by Ficoll-Hypaque density gradient centrifugation of heparinized whole blood. The mononuclear cells were depleted of monocytes by plastic adherence as described previously (Ferrarini et al., 1975). T lymphocytes were obtained from the monocyte depleted population by AET-modified sheep erythrocyte rosetting as previously described (Pellegrino et al., 1975) and separation by Ficoll-Hypaque density gradient centrifugation of the monocyte-depleted mononuclear cells into an E-rosette-positive pellet and an Erosette-depleted interface (Lum et al., 1979). The T cell preparations were routinely tested after their separation and they contained 94-99 (mean 95 5%) E-rosette-positive cells using AET-modified SRBC. The number of sIg-positive cells as Correspondence: Dr L. G. Lum, Metabolism Branch, DCBD, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205, USA. 0099-9104179/0900-0558$02.00 (C 1979 Blackwell Scientific Publications
558
MICCfunction ofhuman T cell subsets
559
determined by anti-human F(ab) 2 specific reagents ranged from 0-1.0%, and the number of cells stained by non-specific esterase prior to culture was less than 3% (Koski, Poplack & Blaese, 1976). T cells that were cultivated overnight at 370C on plastic culture flasks contained less than 055% esterase-positive cells. Isolation of Fc-IgG and Fc-IgM receptor-positive T cells. Rabbit anti-ox erythrocyte IgG and IgM antibodies and the Fc-IgG and FcIgM receptor-positive T cell subpopulations were prepared as described previously (Ferrarini et al., 1975; 1976). The characterization of our reagents by Ouchterlony, immunoelectrophoresis (IEP) and blocking studies with purified IgG and IgM showed that the reagents were specific for the appropriate T cell subsets (Lum et al., 1979). In brief, T cells were rosetted with ox erythrocytes (ORBC) coated with rabbit IgG anti-ORBC (EAIgG) for 20 min at 4VC and separated by Ficoll-Hypaque density gradient centrifugation at 400 g for 40 min. The Ty cell-enriched pellet was lysed with NH4Cl Tris lysing buffer and the interface containing the Ty -depleted cell population was washed and placed into culture along with the washed Ty-enriched cell population. After overnight cultivation at 370C, the Ty-depleted cell populations were rosetted with ORBC coated with rabbit IgM anti-ORBC (EAIgM) for 1 hr at 4VC. Tgi cells were then obtained by lysis of the Tjt-enriched pellet. The Ty-rosetted pellet contained 75-85% rosette-positive cells after a single Ficoll-Hypaque density gradient centrifugation and 88-93% rosette-positive cells after a second centrifugation procedure. The Tj-enriched pellet contained 78-85% rosette-positive cells. Mitogen-induced cytotoxicity assays. The Ty and Tls subpopulations were tested in MICC assays using methods described previously (Muchmore et al., 1975). In brief, whole T, Ty and Tp cells were tested against CRBC or P815Y targets in the cytotoxicity assays using phytohaemagglutinin (PHA) and harvested after 40 hr and 18 hr of incubation, respectively. Cell viability and lymphocyte transformation. Prior to the MICC assays, the cells from the overnight cultivation were counted by trypan blue exclusion and were found to be 90-950% viable. In addition, simultaneous tritiated thymidine incorporation studies were done using PHA and pokeweed mitogen on these cell populations as an independent measure of cell viability. All cell populations gave an adequate response in tritiated thymidine incorporation after 3 and 6 days respectively.
RESULTS MICC to chicken erythrocyte targets In four experiments done to test the MICC capacity of the various T cell populations against CRBC targets, Ty cells were found to be very active killers for CRBC targets with a mean activity of 35 8% "Cr released (Table 1). Tp cells, on the other hand, were only 30%0 as efficient in mediating MICC to CRBC, TABLE 1. MICC of chicken erythrocyte target cells by purified T lymphocyte subpopulations in the presence of PHA T
16-4* 126 178 324 Mean 198
Tp
Ty
28
22 1 235 618 356 358
9.5 6-5 4.7 5.9
* These experiments were done at a target to effector ratio of 1: 1 and were harvested after 40 hr. Each value represents the mean from triplicate experimental tubes. Background release has been subtracted to give a percentage specific release due to PHA stimulation. Spontaneous release was similar for all three sets.
exhibiting a mean activity of 5900 51 Cr released. Whole T cell populations were found to have an intermediate mean activity of 19.8% 51Cr released. In the MICC to CRBC, Ty cells were significantly enriched in cytotoxic activity whilst Tjp cells were depleted of cytotoxic activity for CRBC. The enriched cytotoxic activity of the Ty cell population was present in Ty cell populations prior to and after overnight incubation. Ty cells and Ty-depleted cells assayed in MICC on the first day showed enriched killer activity in the Ty-positive fraction and depletion of killer activity in the Ty negative fraction.
L. G. Lum et al.
560
To explore the MICC activity of these T cell subpopulations further, experiments utilizing multiple effector to target cell ratios were carried out to estimate the relative differences in MICC activity to CRBC (Fig. 1). Ty cells were enriched in cytotoxic activity by approximately twelve times when they were compared to the whole unseparated T cell populations. Purified T9 cells, on the other hand, were essentially devoid of any cytotoxic activity at effector to target ratios from 1-5: 1 to 25: 1.
MICC to DBA/Mastocytoma P8 15 Y tumour cell targets In order to study the cytotoxic behaviour of the same T cell subsets to a different target cell, parallel experiments were conducted using P815Y tumour cell targets. Table 2 presents the results of our experiments. In marked contrast to the results found with CRBC targets, all three of the T cell populations killed P81 5Y target cells with approximately equal efficiency.
50
40~~~~~~~~~
30 -a 40
20
I0
O
5
5
Etfectar FIG. 1. MICC CRBC targets
cells, while 19 25
x10-5 Tpi
for all cell
x
cells
by
T cell
10' unseparated were
populations
at
not
subpopulations.
T cells
cytotoxic
all effector
to
were
to
Iratia
target cel
3500 target cell lysis could be achieved by
required
to
give
a
for CRBC with PHA in this
target cell ratios
was
6-70o
similar
experiment. Background
(0) Ty()
T and
(-) TPt.
TABLE 2. MICC of P815Y plasmacytoma target cells by purified T lymphocyte subpopulations in the presence of PHA T
Ty
TW
25.0*
356 158 141 111 15 3 -5 7
382 133 239 65
174 259 148 Mean 19 2 4J 2 6
1.5
x 101 Ty
degree of target cell lysis. Up
17 7+6 0
* These experiments were done with a target to effector cell ratio of 1 5 1 and were harvested at 18 hr. The data presented are mean values obtained from triplicate determinations. Background release has been subtracted to give a percentage specific 5' Cr release due to PHA stimulation. Spontaneous release was similar for all four sets of experiments.
release of
to
"'Cr
MICCfunction ofhuman T cell subsets
561
DISCUSSION The effector cells of MICC reactions have been extensively studied and it has become apparent that certain cell populations may be quite efficient in killing one type of target cell whilst being totally incapable of lysing another target cell class. For example, T cells, macrophages, polymorphonuclear leucocytes, non-T lymphocytes and even some non-lvmphoid cell lines are efficient killers of CRBC in the presence of PHA. On the other hand, T cells can also lyse P815Y Mastocytoma and Chang cell targets (Blaese, Muchmore & Nelson, 1976; Nelson, Bundy & Strober, 1977). Since it has been established that T cells will mediate MICC (Muchmore et al., 1975), it was not unexpected to find that Ty cells were able to mediate MICC to both P815Y Mastocytoma and CRBC cell targets, but it was unexpected that TV cells were able to lyse only P81 5Y Mastocvtoma targets. This study has demonstrated that there is considerable heterogeneity in the cytotoxic ability of these two human T cell subpopulations. The significance of the capacity of these T cell subpopulations to mediate mitogen- or lectin-induced cytotoxicity reactions remains obscure, although we now have preliminary evidence that such cytotoxic reactions can occur via non-antibody lectin substances found in normal human serum (A. V. Muchmore, unpublished observations). Whether these reactions will prove eventually to be of importance in the host defence mechanism is not known, but differences in reactivity among the T cell subsets as identified bv Fc-receptors for IgG or IgM may offer additional evidence to support the existence of significant functional differences in human T cell subsets. The authors gratefully acknowledge the technical expertise of Mrs Irma Koski in the preparation of the antisera used in this study. REFERENCES
BLAESE, R.M., MUCHMORE, A.V. & NELSON, D.L. (1976) Cellular cytotoxicity reactions induced by mitogens. Mitogens and Immunobiology (ed. by J. J. Oppenheim and D. L. Rosenstreich), p. 443. Academic Press, New York. FERRARINI, M., MORETTA, L., ABRILE, R. & DURANTE, M.L. (1975) Receptors for IgG molecules on human lymphocytes forming spontaneous rosettes with sheep red cell. Europ.]J. Immunol. 5, 70. FERRARINI, M., MORETTA, M.C., TONDA, P. & PERNIS, B. (1976) Human T cell receptor for IgM: specificity for the pentameric Fc fragment. Europ.j. Immunol. 6, 520. GMELING-MEYLING, F., VAN DER HAM, M. & BALLIEUX, R.E. (1976) Binding of IgM by human T lymphocytes. Scand. 3. Immunol. 5, 487. HAYWARD, A.R., LAYWARD, L., LYDYARD, P.M., MORETTA, L., DAGG, M. & LAWTON, A.R. (1978) Fc-receptor heterogeneity of human suppressor T cells.]J. Immunol. 121, 1. KOSKI, I.R., POPLACK, D.C. & BLAESE, R.M. (1976) A nonspecific esterase stain for the identification of monocytes and macrophages. In vitro cell-mediated and tumor immunity (ed. by B. R. Bloom and M. R. David), p. 359. Academic Press, New York. LUM, L.G., MUCHMORE, A.V., KEREN, D., DECKER, J., KOSKI, I., STROBER, W. & BLAESE, R.M. (1979) A receptor
for IgA on human T lymphocytes.]7. Immunol. 122, 65. MORETTA, L., FERRARINI, M., MINGARI, M.C., MORETTA, A. & WEBB, S.R. (1976) Subpopulations of human T cells identified by receptors for immunoglobulin and mitogen responsiveness.]7. Immunol. 117, 2171. MORETTA, L., WEBB, S.R., GROSSI, C.E., LYDYARD, P.M. & COOPER, M.D. (1977) Functional analysis of two human T-cell subpopulations: Help and suppression of B cell responses by T cells bearing receptors for IgM (TM) or IgG (TG). exp. Med. 146, 184. MUCHMORE, A.V., NELSON, D.L., KIRCHNER, H. & BLAESE, R.M. (1975) A reappraisal of the effector cells mediating mitogen induced cellular cytotoxicity. Cell Immuniol. 19, 78. NELSON, D.L., BUNDY, B.M. & STROBER, W. (1977) Spontaneous cell-mediated cytotoxicity by human peripheral blood lymphocytes in itro]. Immuniol. 119, 1401. PELLEGRINO, M.A., FERRONE, S., DIERICH, M.P. & REISFELD, R.A. (1975) Enhancement of sheep red blood cell human lymphocyte rosette formation by the sulfhydryl compound 2-aminoethylisothiouronium bromide. Cliii. Immunol. Immunopathol. 3, 324. PICHLER, W.J., LUM, L.G. & BRODER, S. (1978) Fc-receptors on human T lymphocytes. I. Transition of T gamma to T mu cells.]. Immunol. 121, 1540.
MICC cytotoxic effector function of human T lymphocyte subpopulations bearing Fc-receptors for IgG and IgM L. G. LUM, A. V. MUCHMORE, JEAN M. DECKER & R. M. BLAESE Metabolism Branch, DCBD, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205, USA
(Acceptedfor publication 20 March 1979) SUMMARY
Purified subpopulations of human T lymphocytes bearing Fc-IgG and Fc-IgM receptors were studied for their ability to mediate mitogen (PHA) induced cellular cytotoxicity (MICC) to chicken erythrocyte (CRBC) and DBA/Mastocytoma P815Y tumour cell targets. There were marked differences in the ability of the Fc-IgG receptor-bearing T cell (T y) and the Fc- IgM (Tp) to mediating MICC to CRBC and P815Y target cells. Ty cells were very efficient killers of CRBC and Tji cells had no cytotoxic activity to CRBC. On the other hand, both the Ty and Tlt subpopulations were able to mediate MICC to P815Y tumour cell targets. INTRODUCTION Recently, there has been a great deal of interest in human T lymphocyte subpopulations bearing Fc-IgG and Fc-IgM receptors. Several groups have described Fc-IgG and Fc-IgM receptors on human T cell populations (Ferrarini et al., 1975; Ferrarini et al., 1976; Moretta et al., 1976; Gmelig-Meyling, Van der Ham & Ballieux, 1976). T cells bearing Fc-IgG receptors (T'y) have been demonstrated to be suppressor cells and T cells bearing Fc-IgM receptors (Tj) have been demonstrated to be helper cells in a B cell proliferation assay using pokeweed mitogen (Moretta et al., 1977). More recently, Hayward et al. (1978) have demonstrated that Tji cells exposed to concanavalin A could suppress B cell proliferation. However, Ty cells have also been shown to undergo a transition from a Fc-IgG receptor-bearing cell to a Fc-IgM receptor-bearing cell after IgG immune complex interaction (Pichler, Lum & Broder, 1978). This raises the question of whether or not the Fc-receptors expressed on the various subpopulations of T cells correlate with their functions, especially when their Fc-receptors are being modulated by IgG immune complex interaction. In order to clarify further the functional differences between these two human T cell subsets, we prepared T cell subpopulations enriched for Ty and Tj cells and studied their ability to mediate mitogeninduced cellular cytotoxicity to both chicken erythrocyte (CRBC) and DBA/Mastocytoma P815Y tumour cell targets. MATERIALS AND METHODS Purification of human T cells. Human peripheral blood mononuclear cells were obtained by Ficoll-Hypaque density gradient centrifugation of heparinized whole blood. The mononuclear cells were depleted of monocytes by plastic adherence as described previously (Ferrarini et al., 1975). T lymphocytes were obtained from the monocyte depleted population by AET-modified sheep erythrocyte rosetting as previously described (Pellegrino et al., 1975) and separation by Ficoll-Hypaque density gradient centrifugation of the monocyte-depleted mononuclear cells into an E-rosette-positive pellet and an Erosette-depleted interface (Lum et al., 1979). The T cell preparations were routinely tested after their separation and they contained 94-99 (mean 95 5%) E-rosette-positive cells using AET-modified SRBC. The number of sIg-positive cells as Correspondence: Dr L. G. Lum, Metabolism Branch, DCBD, National Cancer Institute, National Institutes of Health, Bethesda, Maryland 20205, USA. 0099-9104179/0900-0558$02.00 (C 1979 Blackwell Scientific Publications
558
MICCfunction ofhuman T cell subsets
559
determined by anti-human F(ab) 2 specific reagents ranged from 0-1.0%, and the number of cells stained by non-specific esterase prior to culture was less than 3% (Koski, Poplack & Blaese, 1976). T cells that were cultivated overnight at 370C on plastic culture flasks contained less than 055% esterase-positive cells. Isolation of Fc-IgG and Fc-IgM receptor-positive T cells. Rabbit anti-ox erythrocyte IgG and IgM antibodies and the Fc-IgG and FcIgM receptor-positive T cell subpopulations were prepared as described previously (Ferrarini et al., 1975; 1976). The characterization of our reagents by Ouchterlony, immunoelectrophoresis (IEP) and blocking studies with purified IgG and IgM showed that the reagents were specific for the appropriate T cell subsets (Lum et al., 1979). In brief, T cells were rosetted with ox erythrocytes (ORBC) coated with rabbit IgG anti-ORBC (EAIgG) for 20 min at 4VC and separated by Ficoll-Hypaque density gradient centrifugation at 400 g for 40 min. The Ty cell-enriched pellet was lysed with NH4Cl Tris lysing buffer and the interface containing the Ty -depleted cell population was washed and placed into culture along with the washed Ty-enriched cell population. After overnight cultivation at 370C, the Ty-depleted cell populations were rosetted with ORBC coated with rabbit IgM anti-ORBC (EAIgM) for 1 hr at 4VC. Tgi cells were then obtained by lysis of the Tjt-enriched pellet. The Ty-rosetted pellet contained 75-85% rosette-positive cells after a single Ficoll-Hypaque density gradient centrifugation and 88-93% rosette-positive cells after a second centrifugation procedure. The Tj-enriched pellet contained 78-85% rosette-positive cells. Mitogen-induced cytotoxicity assays. The Ty and Tls subpopulations were tested in MICC assays using methods described previously (Muchmore et al., 1975). In brief, whole T, Ty and Tp cells were tested against CRBC or P815Y targets in the cytotoxicity assays using phytohaemagglutinin (PHA) and harvested after 40 hr and 18 hr of incubation, respectively. Cell viability and lymphocyte transformation. Prior to the MICC assays, the cells from the overnight cultivation were counted by trypan blue exclusion and were found to be 90-950% viable. In addition, simultaneous tritiated thymidine incorporation studies were done using PHA and pokeweed mitogen on these cell populations as an independent measure of cell viability. All cell populations gave an adequate response in tritiated thymidine incorporation after 3 and 6 days respectively.
RESULTS MICC to chicken erythrocyte targets In four experiments done to test the MICC capacity of the various T cell populations against CRBC targets, Ty cells were found to be very active killers for CRBC targets with a mean activity of 35 8% "Cr released (Table 1). Tp cells, on the other hand, were only 30%0 as efficient in mediating MICC to CRBC, TABLE 1. MICC of chicken erythrocyte target cells by purified T lymphocyte subpopulations in the presence of PHA T
16-4* 126 178 324 Mean 198
Tp
Ty
28
22 1 235 618 356 358
9.5 6-5 4.7 5.9
* These experiments were done at a target to effector ratio of 1: 1 and were harvested after 40 hr. Each value represents the mean from triplicate experimental tubes. Background release has been subtracted to give a percentage specific release due to PHA stimulation. Spontaneous release was similar for all three sets.
exhibiting a mean activity of 5900 51 Cr released. Whole T cell populations were found to have an intermediate mean activity of 19.8% 51Cr released. In the MICC to CRBC, Ty cells were significantly enriched in cytotoxic activity whilst Tjp cells were depleted of cytotoxic activity for CRBC. The enriched cytotoxic activity of the Ty cell population was present in Ty cell populations prior to and after overnight incubation. Ty cells and Ty-depleted cells assayed in MICC on the first day showed enriched killer activity in the Ty-positive fraction and depletion of killer activity in the Ty negative fraction.
L. G. Lum et al.
560
To explore the MICC activity of these T cell subpopulations further, experiments utilizing multiple effector to target cell ratios were carried out to estimate the relative differences in MICC activity to CRBC (Fig. 1). Ty cells were enriched in cytotoxic activity by approximately twelve times when they were compared to the whole unseparated T cell populations. Purified T9 cells, on the other hand, were essentially devoid of any cytotoxic activity at effector to target ratios from 1-5: 1 to 25: 1.
MICC to DBA/Mastocytoma P8 15 Y tumour cell targets In order to study the cytotoxic behaviour of the same T cell subsets to a different target cell, parallel experiments were conducted using P815Y tumour cell targets. Table 2 presents the results of our experiments. In marked contrast to the results found with CRBC targets, all three of the T cell populations killed P81 5Y target cells with approximately equal efficiency.
50
40~~~~~~~~~
30 -a 40
20
I0
O
5
5
Etfectar FIG. 1. MICC CRBC targets
cells, while 19 25
x10-5 Tpi
for all cell
x
cells
by
T cell
10' unseparated were
populations
at
not
subpopulations.
T cells
cytotoxic
all effector
to
were
to
Iratia
target cel
3500 target cell lysis could be achieved by
required
to
give
a
for CRBC with PHA in this
target cell ratios
was
6-70o
similar
experiment. Background
(0) Ty()
T and
(-) TPt.
TABLE 2. MICC of P815Y plasmacytoma target cells by purified T lymphocyte subpopulations in the presence of PHA T
Ty
TW
25.0*
356 158 141 111 15 3 -5 7
382 133 239 65
174 259 148 Mean 19 2 4J 2 6
1.5
x 101 Ty
degree of target cell lysis. Up
17 7+6 0
* These experiments were done with a target to effector cell ratio of 1 5 1 and were harvested at 18 hr. The data presented are mean values obtained from triplicate determinations. Background release has been subtracted to give a percentage specific 5' Cr release due to PHA stimulation. Spontaneous release was similar for all four sets of experiments.
release of
to
"'Cr
MICCfunction ofhuman T cell subsets
561
DISCUSSION The effector cells of MICC reactions have been extensively studied and it has become apparent that certain cell populations may be quite efficient in killing one type of target cell whilst being totally incapable of lysing another target cell class. For example, T cells, macrophages, polymorphonuclear leucocytes, non-T lymphocytes and even some non-lvmphoid cell lines are efficient killers of CRBC in the presence of PHA. On the other hand, T cells can also lyse P815Y Mastocytoma and Chang cell targets (Blaese, Muchmore & Nelson, 1976; Nelson, Bundy & Strober, 1977). Since it has been established that T cells will mediate MICC (Muchmore et al., 1975), it was not unexpected to find that Ty cells were able to mediate MICC to both P815Y Mastocytoma and CRBC cell targets, but it was unexpected that TV cells were able to lyse only P81 5Y Mastocvtoma targets. This study has demonstrated that there is considerable heterogeneity in the cytotoxic ability of these two human T cell subpopulations. The significance of the capacity of these T cell subpopulations to mediate mitogen- or lectin-induced cytotoxicity reactions remains obscure, although we now have preliminary evidence that such cytotoxic reactions can occur via non-antibody lectin substances found in normal human serum (A. V. Muchmore, unpublished observations). Whether these reactions will prove eventually to be of importance in the host defence mechanism is not known, but differences in reactivity among the T cell subsets as identified bv Fc-receptors for IgG or IgM may offer additional evidence to support the existence of significant functional differences in human T cell subsets. The authors gratefully acknowledge the technical expertise of Mrs Irma Koski in the preparation of the antisera used in this study. REFERENCES
BLAESE, R.M., MUCHMORE, A.V. & NELSON, D.L. (1976) Cellular cytotoxicity reactions induced by mitogens. Mitogens and Immunobiology (ed. by J. J. Oppenheim and D. L. Rosenstreich), p. 443. Academic Press, New York. FERRARINI, M., MORETTA, L., ABRILE, R. & DURANTE, M.L. (1975) Receptors for IgG molecules on human lymphocytes forming spontaneous rosettes with sheep red cell. Europ.]J. Immunol. 5, 70. FERRARINI, M., MORETTA, M.C., TONDA, P. & PERNIS, B. (1976) Human T cell receptor for IgM: specificity for the pentameric Fc fragment. Europ.j. Immunol. 6, 520. GMELING-MEYLING, F., VAN DER HAM, M. & BALLIEUX, R.E. (1976) Binding of IgM by human T lymphocytes. Scand. 3. Immunol. 5, 487. HAYWARD, A.R., LAYWARD, L., LYDYARD, P.M., MORETTA, L., DAGG, M. & LAWTON, A.R. (1978) Fc-receptor heterogeneity of human suppressor T cells.]J. Immunol. 121, 1. KOSKI, I.R., POPLACK, D.C. & BLAESE, R.M. (1976) A nonspecific esterase stain for the identification of monocytes and macrophages. In vitro cell-mediated and tumor immunity (ed. by B. R. Bloom and M. R. David), p. 359. Academic Press, New York. LUM, L.G., MUCHMORE, A.V., KEREN, D., DECKER, J., KOSKI, I., STROBER, W. & BLAESE, R.M. (1979) A receptor
for IgA on human T lymphocytes.]7. Immunol. 122, 65. MORETTA, L., FERRARINI, M., MINGARI, M.C., MORETTA, A. & WEBB, S.R. (1976) Subpopulations of human T cells identified by receptors for immunoglobulin and mitogen responsiveness.]7. Immunol. 117, 2171. MORETTA, L., WEBB, S.R., GROSSI, C.E., LYDYARD, P.M. & COOPER, M.D. (1977) Functional analysis of two human T-cell subpopulations: Help and suppression of B cell responses by T cells bearing receptors for IgM (TM) or IgG (TG). exp. Med. 146, 184. MUCHMORE, A.V., NELSON, D.L., KIRCHNER, H. & BLAESE, R.M. (1975) A reappraisal of the effector cells mediating mitogen induced cellular cytotoxicity. Cell Immuniol. 19, 78. NELSON, D.L., BUNDY, B.M. & STROBER, W. (1977) Spontaneous cell-mediated cytotoxicity by human peripheral blood lymphocytes in itro]. Immuniol. 119, 1401. PELLEGRINO, M.A., FERRONE, S., DIERICH, M.P. & REISFELD, R.A. (1975) Enhancement of sheep red blood cell human lymphocyte rosette formation by the sulfhydryl compound 2-aminoethylisothiouronium bromide. Cliii. Immunol. Immunopathol. 3, 324. PICHLER, W.J., LUM, L.G. & BRODER, S. (1978) Fc-receptors on human T lymphocytes. I. Transition of T gamma to T mu cells.]. Immunol. 121, 1540.
