Restricted helper function of F1 hybrid T cells positively selected to heterologous erythrocytes in irradiated parental strain mice. I. Failure to collaborate with B cells of the opposite parental strain not associated with active suppression.

Unprimed (CBA X C57BL/6)F1 lymph node T cells were transferred with sheep erythrocytes (SRC) into heavily irradiated F1 or parental strain mice and recovered from thoracic duct lymph or spleens of the recipients 5 days later. To study their helper function, the harvested F1 T cells were transferred with antigen into irradiated F1 mice plus B cells from either the two parental strains or from F1 mice. F1 T cells activated in F1 mice gave high IgM and IgG anti-SRC responses with all three populations of B cells. By contrast, F1 T cells activated in mice of one parental strain collaborated well with B cells of this strain, but poorly with B cells of the opposite strain. Active suppression was considered an unlikely explanation for this result since (a) good responses were found with F1 B cells, and (b) addition experiments showed that the poor response with B cells of the opposite parental strain (which was equivalent to that produced by unprimed F1 T cells) could be converted to a high response by a supplemental injection of F1 T cells activated in F1 mice. The phenomenon (a) was specific for the antigen used for activation (criss-cross experiments were performed with horse erythrocytes), (b) was reflected in levels of serum hemagglutinins as well as in numbers of splenic plaque-forming cells, (c) applied also to comparable activation of (DBA/2 X C57BL/6)F1 T cells, and (d) could be prevented by activating F1 T cells in mice of one parental strain in the presence of peritoneal exudate cells of the opposite parental strain. The hypothesis was advanced that F1 T cells contain two discrete subpopulations of antigen-reactive cells, each subject to restrictions acting at two different levels: (a) during T-macrophage interactions and (b) during T-B collaboration. It was suggested that when F1 T cells are activated to antigen in a parental strain environment, radioresistant macrophages activate only one of the two subgroups of T cells, and this subgroup is able to collaborate with B cells of the strain used for activation (and with F1 B cells) but not with B cells of the opposite parental strain. The other subgroup of T cells remains in an unprimed (nonactivated) state.

Restricted helper function of F1 hybrid T cells positively selected to heterologous erythrocytes in irradiated parental strain mice. II. Evidence for restrictions affecting helper cell induction and T-B collaboration, both mapping to the K-end of the H-2 complex.

Studies with H-2-congenic and recombinant strains showed that when F1 hybrid T cells were activated to sheep erythrocytes in irradiated mice of parental strain or related strain, a population of helper cells was generated which collaborated only with B cells sharing the K-end of the H-2 complex with the strain used for activation. No evidence was found that the restriction in helper function (a) reflected a deficiency of appropriate macrophages during T-B collaboration, or (b) was influenced by the Ig allotype of the B cells. It was concluded that the results signified restrictions acting at both the level of helper cell induction (presumed to be a reflection of T-macrophage interactions in the irradiated intermediate hosts) and during T-B collaboration. With (CBA X C57BL/6)F1 cells, the restrictions at each level mapped to the same region i.e. to the left of the I-B subregion. Consequently, one gene (or set of genes) might control restriction at both levels. If so, T-cell recognition of major histocompatibility complex-associated antigen on macrophages and on specific B cells would be either identical or very similar. The fact that genes mapping to the K-end of the H-2 complex also control the restrictive interactions of homozygous T cells implies that F1 T cells behave functionally as a mixture of T cells derived from the two parental strains. Positive selection to antigen in parental strain mice appears simply to alter the ratio of these two populations.

Effects of blocking helper T cell induction in vivo with anti-Ia antibodies. Possible role of I-A/E hybrid molecules as restriction elements.

To examine the role of Ia antigens in controlling T cell activation in vivo, unprimed (CBA X B6)F1 (H-2k X H-2b) T cells were positively selected to sheep erythrocytes (SRC) for 5 d in irradiated F1 mice in the presence of large doses of anti-Iak antibody. With selection in the presence of broad-spectrum anti-Iak antibody (A.TH anti-A.TL antiserum), the activated T cells were markedly reduced in their capacity to collaborate with either B10.BR (I-Ak I-Bk I-Jk I-Ek I-Ck) (kkkkk) or B10.A(4R) (kbbbb) B cells but gave good helper responses with B10 (bbbbb) and (B10 X B10.BR)F1 B cells. Because there was no evidence for suppression, these findings were taken to imply that the anti-Iak antibody bound to Ia determinants on radioresistant macrophagelike cells of F1 host origin and blocked the activation of the IGk-restricted subgroup of F1 T cells but did not affect activation of the Iab-restricted T cell subgroup. Analogous experiments in which F1 T cells were selected to SRC in F1 mice in the presence of monoclonal anti-I-Ak antibody gave different results. In this situation, the reduction in T cell help for Iak-bearing B cells applied to B10.A(4R) B cells but not to B10.BR B cells. With selection of F1 T cells in B10.A(4R) mice, by contrast, anti-I-Ak antibody blocked T cell help for both B10.A(4R) and B10.BR B cells. These data suggested that genes telomeric to the I-A subregion were involved in controlling T cell activation and T-B collaboration. Because no evidence could be found that I-B through I-C determinants per se could act as restrictions elements, the working hypothesis for the data is that Iak-restricted T cells consist of two subgroups of cells: one subgroup is restricted by I-A-encoded molecules, whereas the other is restricted by I-A/E hybrid molecules encoded by two separated genes situated in the I-A and I-E subregions, respectively. The notion that A/E hybrid molecules serve as restriction elements is in line with the findings of other workers that these molecules can act as alloantigens and control responses to certain antigens under double Ir gene control.

Restricted helper function of F1 leads to parent bone marrow chimeras controlled by K-end of H-2 complex.

F1 leads to parent bone marrow chimeras were prepared by transferring F1 hybrid marrow cells into heavily irradiated parental strain mice. When unprimed, donor-derived F1 T cells from the chimeras were activated to sheep erythrocytes (SRC) for 5 days in irradiated normal F1 mice, high IgM and IgG anti-SRC responses were observed with F1 B cells, and with B cells H-2-compatible with the strain in which the T cells were raised from stem cells. Significantly, however, responses with B cells of the opposite parental strain were either absent or very low. The restriction in T-helper function mapped to the K-end of the H-2 complex and could not be attributed to active suppression.

Role of the H-2 complex in induction of T helper cells in vivo. I. Antigen-specific selection of donor T cells to sheep erythrocytes in irradiated mice dependent upon sharing of H-2 determinants between donor and host.

When purified CBA lymph node T cells were mixed with sheep erythrocytes (SRC) and filtered from blood to lymph through irradiated syngeneic mice for 1-2 days, the donor cells lost their capacity to stimulate anti-SRC responses by CBA B cells; the response to a third-party antigen (horse erythrocytes) was unaffected and active suppression was not involved. This process of specific negative selection to SRC also occurred when semiallogeneic mice were used as filtration hosts. By contrast, when allogeneic hosts were used the helper function of the donor cells was not reduced; this applied to both primed and unprimed T cells. Studied with congeneic resistant strains indicated that negative selection to SRC occurred only when the donor and host shared H-2 determinants. Studies with T cells depleted of alloreactive lymphocytes showed that negative selection to SRC in irradiated F1 hybrid mice was followed by a stage of positive selection where the donor cells gave greatly increased responses to the injected antigen. Positive selection did not occur in H-2-different mice, however, and the helper function of the donor cells remained unchanged. By these parameters it was concluded that homozygous T helper cells have no detectable capacity to recognize antigen in an H-2-different environment.

Tolerance of CD8+ T cells developing in parent-->F1 chimeras prepared with supralethal irradiation: step-wise induction of tolerance in the intrathymic and extrathymic environments.

Tolerance of CD8+ cells was examined in parent-->F1 bone marrow chimeras (BMC) prepared with supralethal irradiation; host class I expression in the chimeras was limited to non-BM-derived cells. In terms of helper-independent proliferative responses in vitro and induction of graft-vs.-host disease on adoptive transfer, CD8+ cells from long-term chimeras showed profound tolerance to host antigens irrespective of whether the cells were prepared from the thymus or from spleen or lymph nodes. By limiting dilution analysis, cytotoxic T lymphocyte (CTL) precursors specific for host antigens were rare in the extrathymic lymphoid tissues. In the thymus, by contrast, host-specific CTL precursors were only slightly less frequent than in normal parental strain mice. These host-specific CD8+ cells survived when BMC thymocytes were transferred intravenously to a neutral environment, i.e., to donor strain mice. When transferred to further BMC hosts, however, most of the host-reactive cells disappeared. Collectively, the data suggest that tolerance of CD8+ cells in BMC hosts occurs in both the intrathymic and extrathymic environments. In the thymus, contact with host antigens on thymic epithelial cells deletes CD8+ cells controlling helper-independent proliferative responses and in vivo effector functions but spares typical helper-dependent CTL precursors. After export from the thymus, most of the CTL precursors are eliminated after contacting host antigens on stromal cells in the extrathymic environment.

Specific adsorption of IgM antibody onto H-2-activated mouse T lymphocytes.

Evidence is presented to support the contention that IgM demonstrable by surface immunofluorescent staining on H-2-activated T cells represents specifically adsorbed B-cell-derived alloantibody. T cells activated to H-2 determinants expressed surface IgM only when the progenitor cell populations contained B lymphocytes. IgM was not detected on T cells activated to determinants which fail to stimulate alloantibody production (e.g., M-locus determinants). In addition, IgM-negative H-2 activated T cells (derived from B-cell-depleted lymphoid cells), unlike M-locus-activated T cells, adsorbed IgM in a specific manner when incubated in vitro with "early bleed" antisera raised against the activating H-2 determinants.

Effect of recent antigen priming on adoptive immune responses. III. Antigen-induced selective recruitment of subsets of recirculating lymphocytes reactive to H-2 determinants.

Information was sought on the reactivity of thoracic duct lymphocytes (TDL) from parental strain mice injected intravenously with large numbers of irradiated semiallogeneic spleen cells. TDL collected at 1 day after spleen cell injection were almost totally depleted of lymphocytes able to produce cell-mediated lympholysis (CML), a graft-versus-host (GVH) reaction, and skin allograft rejection against the H-2 determinants on the injected spleen cells. Normal or near normal responses were observed against third-party determinants. In the case of CML, there was no evidence that the unresponsiveness was due to suppressor cells. In marked contrast, the capacity of TDL to exert a specific mixed lymphocyte reaction (MLR) against the injected determinants was reduced by no more than two to fourfold; this applied whether MLR were measured in vivo or in vitro. Injection of normal rather than irradiated semiallogeneic spleen cells gave similar results. Complete and specific removal of MLR-producing lymphocytes was achieved, however, in a different system in which parental strain T cells were filtered from blood to lymph through irradiated F1 hybrid mice. Since this system presumably provided a much higher concentration of H-2 determinants to the responding lymphocytes, it is suggested that the differing results obtained with these two systems may indicate that certain cells reactive to H-2 determinants are of low affinity, their reactivity being detected in the MLR, but not by other parameters. With both systems, MLR-producing lymphocytes reappeared in the lymph after 2-3 days; the cells collected at this stage gave an MLR of altered kinetics. The present data, in toto, suggest that under certain conditions of antigen presentation, virtually all recirculating lymphocytes reactive to a given set of H-2 determinants can be induced to leave the circulation for a period of 1-2 days. After responding to the injected determinants (presumably in organs such as the spleen), the cells re-enter the circulation in an activated state after 2-3 days.

Role of the H-2 complex in induction of T helper cells in vivo. II Negative selection of discrete subgroups of T cells restricted by I-A and I-A/E determinants.

Previous studies have shown that negative selection of T cells to sheep erythrocytes (SRC) after adoptive transfer to irradiated mice requires a sharing of H-2 determinants between the donor T cells and the selection hosts. This paper examines which part of the H-2 complex controls selection. The results show that, in the case of T cells of the H-2k haplotype, complete selection occurs with donor host matching limited to the I-A through I-E subregions of the H-2 complex. Selection to SRC was partial in I-A compatible, I-E incompatible hosts, minimal or not detectable in I-A incompatible, I-E compatible hosts, but near-complete in hosts matched at both the I-A and I-E subregions. Consecutive selection in hosts matched solely at (a) the I-A subregion and (b) the I-E subregion led to incomplete selection. From these and other findings it is argued that H-2k T cells comprise a mixture of T cells restricted by I-A and I-A/E hybrid molecules.

Turnover of naive- and memory-phenotype T cells.

On the basis of their surface markers, T lymphocytes are divided into subsets of "naive" and "memory cells". We have defined the interrelationship and relative life spans of naive and memory T cells by examining the surface markers on murine T cells incorporating bromodeoxyuridine, a DNA precursor, given in the drinking water. Three findings are reported. First, using a new method we show that the release of newly formed naive T cells from the unmanipulated thymus is very low (confirming the findings of others with surgical approaches). Second, in thymectomized mice, T cells with a naive phenotype remain in interphase for prolonged periods; however, some of these cells divide and retain (or regain) their "naive" markers. Third, most T cells with a memory phenotype divide rapidly, but others remain in interphase for many weeks. Collectively, the data indicate that long-lived T cells have multiple phenotypes and contain a mixture of memory cells, naive (virgin) cells, and memory cells masquerading as naive cells.

Absence of H-2 restriction in primary and secondary mixed-lymphocyte reactions to strong M1s determinants.

Negative and positive selection procedures were used to establish whether the strong proliferative response of T cells to M1sa determinants is H-2 restricted. After negative selection of H-2 determinants in vivo, it was shown that T cells give high primary mixed lymphocyte reactions in vitro to M1sa determinants presented on H-2-incompatible stimulator cells. Other studies demonstrated that (a) negative selection of T cells to M1sa determinants on H-2-incompatible cells removed T cells with specificity for M1sa-bearing H-2-compatible cells, and (b) T cells primed in vitro or in vivo to M1sa determinants on H-2-compatible cells gave high secondary responses to M1sa determinants presented either on H-2-compatible or H-2-incompatible stimulator cells. From these data we conclude that T cells recognize M1sa determinants per se rather than an association of M1sa plus self or allo-H-2 determinants.

Selection of cytotoxic T-cell precursors specific for minor histocompatibility determinants. I. Negative selection across H-2 barriers induced with disrupted cells but not with glutaraldehyde-treated cells: evidence for antigen processing.

Intravenous injection of CBA mice with H-2-compatible irradiated B10.BR spleen cells led to a sequence of negative and positive selection of the host T-cell response against the multiple foreign minor histocompatibility antigens (HA) on the injected cells. By 1 d posttransfer, thoracic duct lymphocytes (TDL) of the host had lost the capacity to differentiate in vitro into cytotoxic cells specific for the injected minor HA; spleen and lymph node cells, by contrast, gave normal or enriched responses at this time. By 5 d posttransfer, TDL were hyperresponsive to the injected antigens. Selection with disrupted (sonicated) cells gave similar findings. With injection of either irradiated of disrupted spleen cells, the H-2 haplotype of the minor HA-bearing cells had no apparent effect on the magnitude of selection. By contrast, treatment of spleen cells with glutaraldehyde before injection led to H-2 restriction of selection, i.e., negative selection of the CBA response to B10.BR was marked with injection of glutaraldehyde-treated H-2-compatible B10.BR cells but was minimal with H-2-different B10 or B10.D2 cells. These data are taken to imply that, at least in H-2-incompatible situations, the minor HA-bearing cells must be processed by host cells, i.e., to allow the antigens to become associated with self H-2 determinants. Circumstantial evidence from studies on the specificity of selection induced with glutaraldehyde-treated cells from mice of the B10 recombinant strains suggested that I region-restricted T cells may control the induction of H2K, D-restricted cytotoxic precursor cells.

Negative selection of T cells causing lethal graft-versus-host disease across minor histocompatibility barriers. Role of the H-2 complex.

With a model in which CBA T cells cause lethal graft-versus-host disease (GVHD) in irradiated B10.BR mice (H-2-compatible mice that express multiple minor histocompatibility antigen [HA] differences), information was sought on whether the induction phase of GVHD to minor HA is H-2 restricted. When unprimed CBA (H-2k) T cells were recirculated from blood to lymph for 1 d through irradiated H-2-compatible B10.BR or B10.K mice, the T cells underwent specific negative selection to the minor HA of the host, i.e, the filtered T cells failed to cause GVHD after transfer to B10.BR mice. With filtration through totally H-2-different B10 (H-2b), B10.D2 (H-2d), or B10.S (H-2s) mice, by contrast, no selection occurred, i.e., the filtered cells were unimpaired in their capacity to kill B10.BR mice. Selection was marked after filtration through H-2-semiallogeneic (B10 X CBA)F1 mice. These data, together with the results of filtering T cells through various H-2 recombinant strains, indicated that selection depended upon the donor and filtration host sharing determinants encoded by both the K- and D-ends of the H-2 complex. Compatibility only in the I region failed to cause demonstrable selection.

Resting and activated T cells display different requirements for CD8 molecules.

Clonotype-positive (1B2+) T cells from 2C T cell receptor (TCR) transgenic mice were used to define the role of CD8 molecules in the induction phase vs. the effector phase of the primary response to class I alloantigens. Three main findings are reported. First, in the presence of exogenous lymphokines, resting CD8+ 2C cells gave strong proliferative responses to two alloantigens, Ld and Kbm11. In the absence of added lymphokines, however, CD8+ 2C cells responded only to Ld and not to Kbm11; Ld stimulated both interleukin 2 (IL-2) and IL-2 receptor (R) synthesis, whereas Kbm11 elicited only IL-2R synthesis. The primary response of CD8+ 2C cells was thus helper-independent (HI) to Ld but helper-dependent (HD) to Kbm11, presumably reflecting that Ld is a stronger antigen than Kbm11. Second, in contrast to CD8+ cells, CD8- 2C cells mounted only an HD and not an HI response to the strong Ld antigen; conversely, selecting for a minor subset of CD8hi cells enabled 2C cells to mount an HI response to the weak Kbm11 antigen. These findings, together with experiments with heterozygous vs. homozygous stimulator cells, suggest that HI and HD responses reflect differences in the overall avidity of T antigen presenting cell (APC) interaction: high-avidity interaction leads to strong intracellular signaling and an HI response, whereas low-avidity interaction causes weak signaling and an HD response; high-avidity T/APC interaction is heavily dependent on CD8 expression. Third, CD8 expression was found to be less important for CTL activity than for primary proliferative responses. Thus, in contrast to HI proliferative responses, CTL responses of 2C cells to Ld were CD8 independent. However, 2C lysis of Ld targets became strongly CD8 dependent in the presence of limiting doses of anti-TCR (1B2) antibody. Collectively, the data suggest that, both for T cell induction and the expression of effector function, CD8 molecules play a decisive role in augmenting TCR-mediated signaling, either by promoting TCR contact with antigen or delivering kinases (p56lck) to the TCR/CD3 complex, or both.

Lifespan of gamma/delta T cells.

Information on the turnover and lifespan of murine gamma/delta cells was obtained by administering the DNA precursor, bromodeoxyuridine (BrdU), in the drinking water and staining lymphoid cells for BrdU incorporation. For TCR-gamma/delta (Vgamma2) transgenic mice, nearly all gamma/delta thymocytes became BrdU+ within 2 d and were released rapidly into the peripheral lymphoid tissues. These recent thymic emigrants (RTEs) underwent phenotypic maturation in the periphery for several days, but most of these cells died within 4 wk. In adult thymectomized (ATx) transgenic mice, only a small proportion of gamma/delta cells survived as long-lived cells; most of these cells had a slow turnover and retained a naive phenotype. As in transgenic mice, the majority of RTEs generated in normal mice (C57BL/6) appeared to have a restricted lifespan as naive cells. However, in marked contrast to TCR transgenic mice, most of the gamma/delta cells surviving in ATx normal mice had a rapid turnover and displayed an activated/memory phenotype, implying a chronic response to environmental antigens. Hence, in normal mice many gamma/delta RTEs did not die but switched to memory cells.

Negative selection in the thymus includes semimature T cells.

The thymic medulla plays a key role in negative selection (self-tolerance induction) and contains differentiated T cells en route to the extrathymic environment. However, being relatively mature, medullary T cells are thought to be beyond the stage of tolerance induction. This paradox is resolved by the finding that medullary T cells (CD4+8- thymocytes) comprise two distinct subsets. Medullary thymocytes expressing a fully mature (HSAlo) phenotype are strongly resistant to tolerance induction, whereas cells with a semimature (HSAhi) phenotype are tolerance susceptible. These findings suggest that the differentiated T cells reaching the medulla from the cortex remain sensitive to tolerance induction for a brief period before acquiring a fully mature tolerance-resistant phenotype. The semimature subset of medullarsy T cells displays unique requirements for tolerance induction; depending upon the conditions used, tolerizing these cells can involve either a Fas (CD95)-dependent or a Fas-independent pathway.

Several different cell surface molecules control negative selection of medullary thymocytes.

Repeated attempts to show that costimulation for negative selection is controlled by a single cell surface molecule have been unsuccessful. Thus, negative selection may involve multiple cell surface molecules acting in consort. In support of this idea, we show here that at least three cell surface molecules, namely CD28, CD5, and CD43, contribute to Fas-independent negative selection of the tolerance-susceptible population of heat-stable antigen (HSA)hiCD4+8- cells found in the medulla. The costimulatory function of these three molecules can be blocked by certain cytokines, IL-4 and IL-7, and coinjecting these cytokines with antigen in vivo abolishes negative selection; Fas-dependent negative selection, however, is maintained. The results suggest that efficient negative selection requires the combined functions of at least four cell surface molecules: CD28, CD5, CD43, and Fas.

Influence of antigen dose and costimulation on the primary response of CD8+ T cells in vitro.

The influence of costimulation on the primary response of CD8+ T cells to class I alloantigens was studied with the aid of a T cell receptor transgenic model and defined peptides as antigen. With small doses of antigen, the proliferative response of CD8+ cells was high early in culture but was of brief duration and declined to low levels by day 4; this abbreviated response was associated with limited production of interleukin 2 (IL-2) and was strongly dependent upon costimulation via CD8-major histocompatibility complex class I and CD28-B7 interactions. The response to large doses of antigen was quite different in two respects. First, large doses of antigen inhibited the early (day 3) proliferative response but caused a marked elevation of the response late in culture (day 5); these altered kinetics were associated with increased production of IL-2. Second, the initial proliferative response to large doses of antigen did not require costimulation: indeed, blocking costimulation with CTLA4lg or anti-CD8 monoclonal antibody enhanced the early proliferative response. However, blocking costimulation impaired IL-2 production and prevented the late proliferative response. These findings indicate that the requirement for costimulation of T cells can be partly overcome by increasing the dose of antigen to a high level. However, costimulation plays a key role in prolonging the response, presumably by triggering strong and sustained production of IL-2.

Cell-to-cell interaction in the immune response. VI. Contribution of thymus-derived cells and antibody-forming cell precursors to immunological memory.

Collaboration between thymus-derived lymphocytes and nonthymus-derived antibody-forming cell precursors occurs in the primary antibody response of mice to heterologous erythrocytes and serum proteins. The purpose of the experiments reported here was to determine whether collaboration took place in an adoptive secondary antibody response. A chimeric population of lymphocytes was produced by reconstituting neonatally thymectomized CBA mice soon after birth with (CBA x C57BL)F(1) thymus lymphocytes. These mice could be effectively primed to fowl immunoglobulin G (FgammaG) and their thoracic duct lymphocytes adoptively transferred memory responses to irradiated mice. The activity of these cells was impaired markedly by preincubation with CBA anti-C57BL serum and to a lesser extent by anti-theta-serum. Reversal of this deficiency was obtained by adding T cells in the form of thoracic duct cells from normal CBA mice. Cells from FgammaG-primed mice were at least 10 times as effective as cells from normal mice or from CBA mice primed to horse erythrocytes. These results were considered to support the concept that memory resides in the T cell population and that collaboration between T and B cells is necessary for an optimal secondary antibody response. Poor antibody responses were obtained in irradiated mice given mixtures of thoracic duct cells from primed mice and of B cells from unprimed mice (in the form of spleen or thoracic duct cells from thymectomized donors). In contrast to the situation with T cells, the deficiency in the B cell population could not be reversed by adding B cells from unprimed mice. It was considered that memory resides in B cells as well as in T cells and that priming probably entails a change in the B cell population which is fundamentally different from that produced in the T cell population.

Effect of recent antigen priming on adoptive immune responses. I. Specific unresponsiveness of cells from lymphoid organs of mice primed with heterologous erythrocytes.

When spleen, mesenteric lymph node, or Peyer's patch cells from mice primed 24 h before with either sheep erythrocytes (SRC) or horse erythrocytes (HRC) were transferred together with both SRC and HRC to irradiated mice, antibody responses measured 7 days later were very low to the priming antigen but high to the other antigen. This was demonstrated either by measuring numbers of antibody-forming cells in spleen or levels of hemagglutinins in serum. Specific unresponsiveness of the transferred cells was evident in both the 19S and 7S responses. It was observed only when strict experimental conditions were followed: (a) the cell donors had to be primed with not less than 10(9) erythrocytes given intravenously; (b) the cells had to be transferred between 1 and 2 days after antigen priming; (c) antibody responses in the recipients were measured within 7 days of cell transfer, i.e., partial recovery was evident by 11 days; (d) the transferred cells had to be challenged in the recipients within 1 day after cell transfer: when challenge was delayed for 5 days or longer, responsiveness returned. The failure of cells from recently primed donors to respond to the priming antigen on adoptive transfer could be overcome by supplementing with normal spleen cells, but not with thymus alone or bone marrow alone. This implied that unresponsiveness occurred at the levels of both T and B lymphocytes, and was not due to a suppressive influence exerted by T cells. Further work is in progress to determine the mechanism of this transient state of specific unresponsiveness.