Modulation of regulatory mechanisms operative in the cyclical production of antibody.

Modulation of the cyclical response in rabbits to aggregated human gamma globulin (AHuIgG) was investigated in order to study some of the parameters involved in self-regulation of the immune response. Several mitogens (lipopolysaccharide [LPS], phytohemagglutinin [PHA], and concanavalin A [Con A]), when injected simultaneously with antigen, have been shown to modulate the normal splenic plaque-forming cell (PFC) response in rabbits to a single intravenous injection of AHuIgG. This response to AHuIgG has previously been characterized by the initial appearance of PFC in the spleen 3 days later, with a peak of PFC at 5 days after injection. The number of PFC in the spleen then decreases and remains at a low level until a second increase begins on day 10, peaking on day 13. The 8-day cycle between peak PFC repeats, with a third peak appearing on day 21. In the present studies, injection of LPS with AHuIgG was shown to affect the PFC response by enhancing only the initial peak of PFC, PHA was shown to enhance both the initial and secondary peaks of PFC, while injection of Con A with AHuIgG resulted in a prolonged increase in PFC with no apparent cycling. Irradiation 24 h after injection of antigen resulted in PFC kinetics similar to those observed with PHA, although the increase in PFC was more marked with irradiation. Thus, although LPS, PHA, Con A, and irradiation markedly affected the immune response to AHuIgG, Con A was the only substance which altered the cyclical appearance of PFC to HuIgG. The cyclical nature of the PFC kinetics was shown to occur with either intravenous or intraperitoneal injection of antigen and in both primary and secondary responses, provided that the rabbits were primed with a low dose of antigen. Data were obtained that suggest that the response in distal lymph nodes may be regulated by immunological events occurring in the spleen. Cycling of PFC was not observed in the draining node after subcutaneous injection of AHuIgG in the hind foot. However, if the antigen was also injected intravenously at the same time as the subcutaneous injection, the response in the node became cyclical.

The requirement for adherent cells in the Fc fragment-induced proliferative response of murine spleen cells.

The proliferative response of mouse B lymphocytes induced by Fc fragments was found to be dependent upon an adherent cell population. The adherent cell is esterase positive, irradiation resistant, and not susceptible to lysis by anti-thymus serum and complement. The mechanism(s) by which Fc fragments induce B-cell proliferation could be the result of the interaction of Fc with both B cells and adherent cells or with adherent cells which then release factors that trigger the B cells to proliferate. Spleen cells from the C3H/HeJ mouse were shown to be unable to respond to Fc fragments. The addition of adherent cells from either C3H/St or C3H/HeN mice to adherent cell depleted C3H/HeJ cells enabled them to respond to Fc, indicating the defect was in the adherent cell population.

Regulation of Fc fragment-induced murine spleen cell proliferation.

Murine splenic lymphocytes proliferate in response to supernatant material derived from Fc fragment-pulsed splenic adherent cells. The stimulatory supernatant results from the interaction of Fc fragments with adherent cells or adherent cell supernate. Isolation of the stimulatory material in the supernate by Sephadex chromatography revealed that the mitogenic component was a cleavage product of Fc with a mol wt of approximately 14,000. The spleen cell type responsible for the generation of mitogenic Fc subfragments appears to be a macrophage. Unstimulated macrophages release an active supernate without being exposed to Fc fragments. The supernate of unstimulated macrophages apparently contain an enzyme which is capable of cleaving Fc fragments into the 14,000-mol wt mitogenic molecules. The spleen cell population induced to proliferate in response to the adherent cell supernate is present in T-cell depleted and Sephadex G-10 filtered cell preparations. Depletion of cells bearing immunoglobulin on their surfaces results in a reduced proliferative response to the mitogenic supernatant material indicating that it is probably a B cell.

In vivo induction of tolerance in murine CD4+ cell subsets.

The induction of tolerance in mice to preparations of deaggregated human gamma globulin (DHGG) results in in vitro antigen-specific unresponsiveness in CD4+ T cells as well as in both the T helper 1 (Th1) and Th2-like subpopulations. Whereas both CD45RB(hi) and CD45RB(lo) cells from lymph nodes of HGG/complete Freund's adjuvant-immunized mice (control) proliferated in vitro to HGG, both subpopulations from mice previously tolerized with DHGG failed to respond. Furthermore, CD4+ T cells from control, but not from DHGG-injected mice, secreted high levels of interleukin 2 (IL-2) after in vitro stimulation with HGG. Although significant levels of IL-4 in supernatants of control CD4+ cells stimulated with HGG were detected in some, but not all, experiments, significant levels of IL-4 were never detected in supernatants of HGG-stimulated tolerant CD4+ cells. The demonstration that serum IgG1 anti-HGG is preferentially produced in a few tolerant mice that exhibit a leaky tolerant state suggests that tolerance induction may be more difficult to induce in IL-4- than in IL-2-producing cells.

Fate of antigen-binding cells in unresponsive and immune mice.

Antigen-binding cells (ABC) to the antigen human gamma globulin (HGG) were quantitated in lymphoid tissues of A/J mice at various times after the injection of deaggregated HGG (tolerogen), aggregated HGG (immunogen), or saline. The reaction of lymphoid cells with highly labeled HGG was specific to that antigen since binding could be inhibited by excess unlabeled HGG, but not by unrelated non-cross-reacting proteins. Compared with normal mice, there was a marked decrease in the numbers of ABC in the spleens of unresponsive animals evident as early as 12 h after the injection of tolerogen. A marked increase in ABC was observed in the spleens of immunogen-injected mice, beginning at 24 h and reaching a peak at 3 days. In bone marrow, no difference in the number of ABC was found among the three experimental groups until day 20, when a reduction in ABC was observed only in tolerogen-injected mice. No quantitative difference in the thymuses in the experimental groups could be determined because of the paucity of ABC displayed by normal thymus cells.

Termination of tolerance to human gamma globulin in mice by antigen and bacterial lipopolysaccharide (endotoxin).

Bacterial lipopolysaccharides (endotoxin) allowed the circumvention of the thymus-derived (T) cell helper function otherwise required for the antibody response in mice to human gamma globulin (HGG). In an analogous fashion, the state of tolerance to HGG, existing at a time when bone marrow-derived (B) cells had lost their unresponsiveness, could be terminated by the injection of both immunogenic HGG and endotoxin, but by neither given alone. However, no effect on tolerance to HGG could be observed when this regimen was followed at a time when B cells were tolerant. After the spontaneous recovery from tolerance in B cells, it seemed that specific priming was occurring in that population. This phenomenon was observed either by the injection of endotoxin and HGG or by the adoptive transfer of cells into irradiated hosts. These data have been discussed in the light of potential autoimmune manifestations that could theoretically follow a simultaneous gram-negative bacterial infection along with a release of self-antigen.

Stimulation of antibody production to the hapten 2,4-dinitrobenzene by affinity labeled murine lymphoid cells. I. Ability of affinity labeled murine lymphoid cells to activate the in vivo immune response.

The ability of meta-nitrobenzenediazonium fluoborate (m-NBDF)-labeled thymus and spleen (S) cells to transfer immunity to 2,4-dinitrophenyl (DNP) into irradiated syngeneic recipients was investigated. There was a significant increase in the number of anti-DNP plaque-forming cells (PFC) when m-NBDF-labeled thymus cells and normal spleen cells, or normal thymus cells and m-NBDF-labeled spleen cells were transferred, but not when both thymus- and S-cell populations were labeled and injected together into irradiated recipients. The ability of these cell populations to cooperate and enhance the in vivo immune response to DNP is discussed. The T cells seem to be actively involved in the development of this response; they participate beyond the mere role of carrying and presenting antigen to the B cells. It is suggested that cell to cell contact between T and B cells may be an important factor in the elicitation of an immune response. In addition, the cellular interaction is affected by irradiating the thymus cell preparation and the initiating interaction required for antibody synthesis probably occurs within 48 h after injecting the cell populations into the syngeneic irradiated recipients.

A cyclical appearance of antibody-producing cells after a single injection of serum protein antigen.

After a single intravenous injection of rabbits with aggregated HuIgG, IgM- and IgG-plaque-forming cells (PFC) in both the spleens and peripheral blood of rabbits peaked 5, 13, and 21 days after injection, while almost no PFC could be detected on days 8 and 16. The available data suggest that the secondary peaks of PFC (days 13 and 21) resulted from stimulation of memory cells by persisting antigen that was localized in the germinal centers in the spleen. No such persistence of antigen occurred in the lymph nodes, and these lymphoid tissues did not exhibit secondary peaks of PFC. The identical kinetic patterns for IgM- and IgG-PFC indicate that the major portion of IgG-PFC did not result from IgM-secreting cells switching to IgG synthesis and secretion. The present data suggest that the antibody produced and present at the site of interaction between committed cells and antigen is responsible for the regulation of antibody synthesis to persisting antigens. Possible cellular events involved in both the regulation and an apparent synchronous appearance of antibody producing cells in the spleens of rabbits were presented.

Roles of T and B lymphocytes in the termination of unresponsiveness to autologous thyroglobulin in mice.

The data presented in this paper support the hypothesis that unresponsiveness to autologous thyroglobulin (Tg) exists in the T cells and responsiveness exists in the B cells. Such a conclusion is based on the results of antigen-binding studies where few, if any, thymocytes recognized syngeneic Tg. Comparable numbers of antigen-binding lymphocytes for syngeneic Tg were found in the spleens of normal intact mice and of nude mice. The latter fact suggested that B cells exist which recognize self-constituents. From antigen-suicide experiments, a clearer picture of the susceptibility of B cells to iodinated self-antigen and of the obligatory role of antibody in the induction of lesions was developed. Only bone marrow cells (B cells) were affected by [(125)I]syngeneic Tg, in which case the incidence of lesions was diminished. From adoptive transfer experiments, the results demonstrate that unresponsiveness may be terminated by immunization with a mixture of heterologous (cross-reacting) Tg's. In this situation T cells are required since a B-cell reconstituted host failed to make antibody (plaque-forming cells) and to develop lesions. T cells in this form of an unresponsive state may recognize determinants on the heterologous Tg unrelated to autologous Tg and as such stimulate the normal complement of B cells to produce antibody that both reacts with autologous and heterologous Tg.

Tolerogenicity of resting and activated B cells.

Antigen presentation by resting splenic B cells has been shown previously to induce T helper 1 cell (Th1) anergy. In contrast to expectations, it was found here that B cells treated with F(ab')2 goat anti-mouse immunoglobulin (IgM) for 24 or 48 h also presented antigen (Ag) to Th1 cells in a manner that induced dramatic Ag-specific proliferative inactivation. The tolerogenicity of the anti-Ig-treated B cells was consistent with the observation that these B cells were only slightly more efficient than resting B cells in stimulating human gamma globulin (HGG)-induced proliferation of HGG-specific Th1 cells in primary cultures. The activated B cells were, however, more efficient than resting B cells in stimulating a primary mixed leukocyte reaction, and exhibited increased expression of major histocompatibility complex class II molecules, RL388 Ag and transferrin receptor. In addition, unlike resting B cells, which expressed little detectable B7, anti-Ig-treated B cells expressed high levels of B7. The functional capacity of the B7 expressed on the activated B cells was demonstrated by the fact that the Ag-presenting capacity of these B cells was inhibited by the addition to culture of CTLA4Ig, a soluble receptor for B7. It is unlikely that the tolerogenicity of the activated B cells was due to an inability of the Th1 cells to respond to B7 signals; the Th1 clones used in the experiments, unlike the Th2 clones tested, expressed CD28, the ligand for B7. In addition, anti-CD28 monoclonal antibody inhibited the induction of Th1 cell anergy when added to cultures of Th1 cells and Ag-pulsed fixed antigen-presenting cells. Taken together, the results indicate that B cells, even when activated, do not satisfy the costimulatory requirements of the Th1 cells used here, and therefore can present Ag in a tolerogenic fashion to Th1 cells. The costimulator deficiency of activated B cells may reflect an inadequacy in the level of B7 expressed or a lack of some other molecule.

The in vivo formation and fate of antigen-antibody complexes formed by fragments and polypeptide chains of rabbit gamma-G-antibodies.

The in vivo formation and subsequent fate of complexes formed between specific rabbit gammaG-antibody subunits and circulating protein antigens was studied in rabbits and guinea pigs. Subunits obtained from purified antibodies were injected immediately after an injection of antigen, and the elimination from the circulation of either I*-labeled gammaG-subunits or labeled antigen determined. In the absence of antigen, all gammaG-subunits which lack the Fc fragment were rapidly eliminated. In the presence of excess antigen, F (ab')(2), Fab and Fd fragments reacted with antigen and remained in the circulation as complexes which were eliminated at the same rate as the antigen. Fab hybrids containing a specific Fd fragment and a nonspecific L chain similarly reacted with antigen and remained in the circulation complexed to antigen. In contrast, L chains and Fab hybrids containing a specific L chain and a nonspecific Fd fragment did not react with antigen in vivo and were rapidly eliminated in both presence and absence of antigen. H chains remained in the circulation of rabbits in the absence of antigen, however, in the presence of antigen, more H chains which formed complexes with antigen remained in the intravascular space and were rapidly eliminated when the immune elimination of the antigen by the host occurred. Nonspecific H chains were rapidly eliminated from the circulaion of guinea pigs, whereas specific H chains remained in the circulation with the antigen. F (ab')(2) fragments formed complexes with antigen near antibody equivalence and in antibody excess which were rapidly eliminated, however, less effectively than complexes formed near antibody equivalence with intact gammaG. Complexes formed in antibody excess between Fab fragments and H chains remained in the circulation at all concentrations studied and were eliminated at the rate of antigen. The role of the Fc fragment in the immune elimination of antigen-antibody complexes is discussed.

Nonspecific activation of murine lymphocytes. I. Proliferation and polyclonal activation induced by 2-mercaptoethanol and alpha-thioglycerol.

The effect of 2-mercaptoethanol (2-ME) and alpha-thioglycerol (alpha TG) on proliferation and polyclonal activation of lymphocytes was studied in cultures of spleen cells from C3H mice. Inclusion in serum-free or serum-containing medium of the optimal concentration (5 x 10(-5) M) of either 2-ME or alpha TG resulted in highly significant uptake and incorporation of tritiated thymidine ([3H]TdR) into DNA and in morphological blast transformation. These phenomena were dose-dependent, with both lower and higher doses causing less marked effects. The kinetic peak of these responses was found to occur at day 3 of culture. Improved cellular viability could not explain these results, because by day 3 there was no significant difference in viability between cells cultured in the presence or absence of 2-ME. 2-ME evoked a proliferative response in cultures of congenitally athymic (nu/nu) spleen cells that exhibited a similar but lower dose-response profile compared with that of heterozygous (nu/+) littermates. Cultures of bone marrow-derived (B) lymphocytes, generated by treatment of spleen cells with rabbit antithymocyte serum and complement, incorporated [3H]TdR to a degree at least equal to that of normal spleen cell cultures. Thymus-dependent (T) cells did not support significant 2-ME, alpha TG, or Concanavalin A responses in the absence of serum. However, when cultured in 5% fetal calf serum, definite T-cell responses occurred, though always of a lower magnitude than B-cell responses in this system. When the enriched B-cell and T-cell preparations were co-cultured, a synergistic response was noted. Macrophage dependency of the 2-ME and alpha TG effect was shown to be minimal. It is likely that the greater effectiveness of alpha TG relative to 2-ME is due to differences in the chemical structure of these two thiol compounds. The advantages of utilizing 2-ME and alpha TG as probes in the study of lymphocyte activation are evaluated and their possible mechanisms of action are discussed.

B-lymphocytes activation by the Fc region of IgG.

Strong stimulation of DNA synthesis (up to 150-fold) and blast transformation can be induced in mouse spleen cells by Fc fragments of human IgG. The mitogenic response is optimal on day 5 of culture and is dependent on the concentration of Fc fragments with a sedimentation rate of 3-5S. Intact IgG is also stimulatory, but only when modified by heat aggregation, and produces only a 10-fold increase in [3H]thymidine uptake. The stimulation by aggregated IgG is dependent on the Fc portion, since aggregated (or soluble) Fab or F(ab')2 fragments are inactive. The results show that the response is T-cell independent and that it is a function of nylon wool adherent, surface Ig-positive, Fc receptor-bearing B lymphocytes. Fc fragments do not induce plaque-forming cells to human IgG in normal mouse spleen cell cultures, but rather trigger polyclonal antibody synthesis (anti-goat erythrocytes, anti-2,4,6-trinitrophenyl). It is postulated that the Fc region of antibodies plays a role in the regulation of the humoral immune response by triggering clonal expansion of B lymphocytes.

Cellular events in the induction of experimental allergic encephalomyelitis in rats.

Although both the T and B cells of the Lewis rat have immunoglobulin receptors for basic protein (BP) of myelin, and both cell types are required for antibody production to BP, the present results demonstrate that the T cells are the only cells required for the induction of experimental allergic encephalomyelitis (EAE). Both EAE and anti-BP were readily induced in thymectomized, irradiated Lewis rats reconstituted with normal thymus and bone marrow cells and challenged with BP in complete Freund's adjuvant. If the thymus cells were first treated with BP heavily labeled with 125I so as to eliminate (sucide) specific T cells, the recipients neither develop EAE nor produce antibody to BP. On the other hand, if the thymus cells were untreated and the specific B cells of bone marrow were eliminated by treatment with 125I-BP, EAE was not inhibited, although no antibody was produced. These results strongly suggest that the T cell is responsible for the induction of EAE although both the T and B cells are competent to respond to BP. Evidence was presented which suggests that neither suppressor T cells nor circulating antibody are involved in the inhibition of EAE by injection of Lewis rats with nonencephalitogenic preparations of BP. The immune status of T and B cells of the Lewis rat to BP was compared with the immune status of these cells in other species to thyroglobulin, where only the B cells appear to be competent. In this context, Brown Norway rats, which are resistant to the induction of EAE, also appear to lack T cells reactive to BP, although competent B cells are present.

Transfer of experimental autoimmune thyroiditis by serum from thyroidectomized donors.

When rabbits were injected with 10.0 mg rabbit thyroglobulin in complete Freund's adjuvant, the earliest thyroid lesions were seen on day 5 and uniformly severe thyroid lesions were seen by day 14; these observations were not significantly different from the thyroid lesions observed at 1 and 2 months post-immunization. Pooled sera were obtained from immunized, thyroidectomized, and nonthyroidectomized donors on various days and transferred to normal recipient rabbits in different experiments. Successful transfer of thyroid lesions was seen when serum containing early antithyroglobulin antibody obtained from thyroidectomized donor animals at various times after immunization was injected into normal recipients in a sequential manner. Immunofluorescent studies of recipient thyroid glands showed focal fixation of rabbit gamma-globulin and beta(1C) complement in thyroid follicles. When purified antibody to rabbit thyroglobulin obtained from thyroidectomized donor sera was transferred sequentially as above, significant thyroid lesions were seen in recipient rabbits. In contrast, no thyroid lesions were seen in recipient animals injected with rabbit sera containing late antithyroglobulin antibody from thyroidectomized donors or hyperimmune sera from guinea pigs. No thyroid lesions were seen in recipient animals injected either with sera from donors given complete adjuvant without thyroglobulin or with globulin fraction of pooled sera containing early antithyroglobulin antibody obtained on various days from nonthyroidectomized donors. Similarly, rabbits rendered unresponsive to guinea pig gammaG-globulin and periodically injected with guinea pig anti-rabbit thyroglobulin showed no thyroid lesions.

The termination of immunological unresponsiveness to bovine serum albumin in rabbits. I. Quantitative and qualitative response to cross-reacting albumins.

Rabbits made unresponsive to BSA at birth were given two courses of immunization with various cross-reacting albumins at 3 months of age. Normal control rabbits, of equivalent age and weight, were similarly immunized. Sera obtained 7 days after the last injection were assayed for binding and precipitating antibody to six albumins and for their avidity for BSA. No significant differences were found between unresponsive and normal rabbits in the amount of antibody reacting with any of the six albumins used. This was the case regardless which albumin was used to terminate the unresponsive state. Avidity differences were seen and seemed to depend on the antigen used and not on the immunological status of the animal. The simultaneous injection of small amounts of BSA inhibited the termination of unresponsiveness. These results were discussed in the light of the more recent theories of the termination of unresponsiveness and of antibody formation.

Polyclonal activation of human B lymphocytes by Fc fragments. I. Characterization of the cellular requirements for Fc fragment-mediated polyclonal antibody secretion by human peripheral blood B lymphocytes.

Fc fragments derived from human immunoglobulin were found to be capable of inducing both a proliferative and polyclonal antibody response in human peripheral blood lymphocyte cultures. The cell population proliferating in response to Fc fragments belongs to the B cell lineage. Expression of polyclonal antibody formation requires the presence of both adherent monocytes and T cells. The role of the monocyte is to enzymatically cleave the Fc fragment into 19,000 mol wt Fc subfragments that are then able to induce polyclonal antibody secretion. Stimulation of polyclonal antibody production by Fc subfragments occurs in the absence of adherent monocytes but still requires the presence of T cells.

T cell regulation of polyclonal B cell responsiveness. III. Overt T helper and latent T suppressor activities from distinct subpopulations of unstimulated splenic T cells.

Polyclonal activation of murine splenic B lymphocytes by lipopolysaccharide was found to be subject to regulation by helper and suppressor influences from T lymphocytes. In the normal adult spleen, only helper influences were exercised over polyclonal B cell activation; this influence is a property of Lyt-l(+)23(-) slowly sedimenting T cells. Suppressive influence evidently is latent, for it exists at such a low level (or the cells are so few in number) that its effects are difficult to detect. Suppressor T cell function may be evoked by culturing spleen cells at high ratios of T:B cells, by activating splenic T cells with concanavalin A, or by sonicating unstimulated splenic T cells to liberate a suppressive potential that is not expressed by these unstimulated cells when intact. The soluble fraction of resident splenic T cell sonicates exerts both helper and suppressor regulatory influences. The soluble helper activity is derived from Lyt-l(+)23(-) slowly sedimenting T cells, whereas suppressor activity is generated from a distinct subpopulation of Lyt-l(-)23(+) rapidly sedimenting T cells. The thymus contains cells capable only of helping but not of suppressing polyclonal activation of splenic B cells. Helper and suppressor activities contained in splenic T cell sonicates were separated by gel chromatography; the suppressive activity was found to elute with a molecular weight between 68,000 and 84,000 and the helper activity eluted with a molecular weight between 15,000 and 23,000. The data indicate that helper and suppressor activities are distinct molecular entities derived from distinct splenic T lymphocyte subpopulations. The possibility that these molecules are precursors to or components of antigen- specific or nonspecific helper and suppressor factors described in the literature is discussed.

In vitro tolerance induction of primed, IgD-negative murine spleen cells.

The above observations demonstrated induction of immunological tolerance in vitro in primed IgD-, IgG+ B cells. In these studies, addition of trinitrophenylated (TNP) turkey gammaglobulin (TGG) or TNP ovalbumin conjugates suppressed the secondary in vitro response in mice primed with TNP keyhole limpet hemocyanin (TNP-KLH). Suppression was not a reflection of a shift in kinetics of the antibody response, was not dependent on suppressor T cells, and could only be eliciate when conjugate was added within 4 h of addition of TNP-KLH moreover, preincubation of the primed spleen cells with TNP-TGG for 20 h at 37 degrees C, followed by extensive washing, was as effective in inhibiting the response to TNP-KLH as when TNP-TGG was present throughout the 5 d of culture, reflecting induction of a tolerant state. Amounts of conjugate in the concentration range that have been shown by others to tolerize immature or neonatal B cells or mature B cells that have been stripped of surface IgD were sufficient to induce tolerance. The target cells being tolerized did not bear IgD, as determined by B cell depletion and blocking procedures with anti IgD. Whether the lack of surface IgD on the primed cells contributed to the relative ease of tolerance induction was not established by these studies, but the advantages of using primed B cells to examine further the role of surface IgD in tolerance susceptibility was discussed.

Nonspecific activation of murine lymphocytes. VI. Mediation of synergistic interaction between T and B lymphocytes by a cell-associated, reciprocally acting lymphocyte proliferation helper.

The mechanism by which B and T lymphocytes interact synergistically in the proliferative response to 2-mercaptoethanol (2-ME) as a mitogen was investigated in cultures of C3H/St spleen cells. The interaction between these cells required physical contact between the collaborating cell types, and was not mediated by the release of a soluble factor into the culture supernate. Sonicates of spleen cells which had been activated with optimal concentrations of 2-ME for 24 h and then washed extensively, stimulated the uptake of tritiated thymidine and morphological blast transformation of fresh, unstimulated cells. This activity was found to reside within the soluble fraction of the activated cells, and to activate cells optimally at a ratio of 1 naive cell: 1 activated cell-equivalent. This reciprocally-acting lymphocyte proliferation helper (RALPH) activity was produced by B cells as well as by T cells, with a kinetic peak at 48 h of culture. RALPH activity was produced by viable but not by nonviable cells incubated with 2-ME, and was nondialyzable. It could not be induced by the B-cell mitogens lipopolysaccharide, polyinosinic-polycytidilic acid, or purified protein derivative of tuberculin, or by the T-cell mitogen concanavalin A. RALPH isolated from T cells activated B cells exclusively, while that from B cells acted predominantly upon T cells, possibly with a nonspecific effect on B cells. A model for the cellular interactions involved in the amplification of the proliferative response to 2-ME is described.