Proliferation of murine thymic lymphocytes in vitro is mediated by the concanavalin A-induced release of a lymphokine (costimulator).

Mitogen-induced proliferation of lymphocytes may in theory result directly from the interaction of mitogen with the cells, or indirectly as a result of the mitogen-stimulated release of lymphokines. In the case of murine thymic lymphocytes exposed to concanavalin A (Con A) in tissue culture, we have determined that mitogenesis depends upon a lymphokine. Interaction of the thymic lymphocytes with lectin is necessary, but not sufficient, for mitogenesis. A lymphokine, or costimulator for mitogenesis, is released by normal spleen or thymus cells during the first 16 hr of their exposure to Con A, and in the presence of a phytomitogen it stimulates thymic mitogenesis. Under conditions of low costimulator levels, no mitogenesis follows the interaction of Con A with cells. The response of adult CBA/J mouse thymocytes to phytohemagglutinin (PHA) is very low, compared to their response to Con A. When costimulator is added to PHA, the cells respond as well as they do to Con A. Costimulator does not act through Con A-binding sites on thymus cells. Its production is dependent on both cells carrying omega surface antigen (T lymphocytes) and adherent cells of the macrophage-monocyte series. The adherent population, but not the T cells, may be heavily irradiated without affecting production of costimulator. Costimulator is not a mitogen on its own.     

The role of macrophages in thymocyte mitogenesis.

The thymic lymphocytes of CBA/J mice respond to the mitogen Concanavalin A (Con A) only in the presence of adherent cells of the monocyte-macrophage series. Depletion of adherent cells abrogates the response, and macrophage-rich population of cells restore it. The need for macrophages and mitogen is completely provided by irradiated splenic macrophages which have been exposed to Con A and washed free of the soluble mitogen. The mitogenmacrophage effect in this case is apparently not due to soluble factors. Even more striking than the effect of macrophages on fresh cultures of thymic lymphocytes is their ability to restimulate quiescent cells 72 hr after their first stimulation with Con A. The quiescent cells respond immediately and quantitatively to Con A in the presence of fresh macrophages. This stimulation, like that of fresh thymocytes, is also controlled by a lymphokine ("costimulator") produced by mixing macrophages, mitogen, ant T lymphocytes. Our data suggest a model in which two signals are required for mitogenesis. First, the interaction of macrophage, T cell, and mitogen elicits a soluble costimulator, which is itself not mitogenic. Secondly, in the presence of costimulator, the mitogen (either soluble, or, more efficiently, bound to macrophages) induces a proliferative response in the T cell.     

A spleen-derived maturational factor allows immature thymocytes, prepared as cells bearing low amounts of surface sialic acid, to become cytotoxic T cells

A population of immature mouse thymocytes bears low levels of surface sialic acid and can be separated from the more mature high sialic acid-bearing thymocytes by selective agglutination with the sialic acid-specific lectin, lobster agglutinin 1. These immature thymocytes do not proliferate in response to concanavalin A (Con A). They do not produce interleukin 2 (IL-2), do not provide T cell help to B cells for an in vitro antibody response, and as shown here, do not become cytotoxic T lymphocytes when polyclonally stimulated with Con A + IL-2. We describe here a spleen-derived maturational factor which stimulates these immature thymocytes, in the presence of Con A and IL-2, to become cytotoxic T lymphocytes. The maturational factor is a protein secreted by Con A-stimulated mouse or rat spleen cells; it is apparently neither interleukin 1, IL-2, interleukin 3, gamma-interferon, nor combinations of these cytokines, because these materials do not replace the maturational factor. The active material in Con A-stimulated mouse spleen cell supernatant was recovered from a G-75 column in the 33,000-48,000 m.w. range. These experiments suggest that within the lobster agglutinin 1-negative thymocyte population there are cells which can mature under the influence of a spleen-derived factor. It is possible that these cells represent the small subpopulation of immature cells destined to become immunocompetent peripheral T cells. On the other hand, the factor may be rescuing cells destined to die in the thymus.     

Partial purification and molecular characterization of a lymphokine (costimulator) required for the mitogenic response of mouse thymocytes in vitro.

We have partially purified a lymphokine, costimulator, which is necessary to induce mitogenesis in mouse thymocytes in vitro. Costimulator is released from mouse leukocytes exposed to Con A for 12 to 18 hr. It has been purified more than 100 X by gel exclusion chromatography and isoelectric focusing. Thymocytes from CBA/J mice respond to the mitogenic lectin Con A only if the costimulator concentration is above a certain level. Culturing such cells with Con A at a density below 1 X 10(6) cells/ml produces costimulator concentrations too low for mitogenesis. This system has been developed into a quantitative assay for costimulator, to monitor purification, recovery, and biologic activity in various methods of molecular characterization. The activity is trypsin sensitive, and has a buoyant density characteristic of protein or glycoprotein. However, for a protein, it is relatively heat stable. Its m.w., established by carrying out sedimentation, gel filtration, and buoyant density measurements, is 30,500, and its frictional coefficient is 1.45. Costimulator purified by isoelectric focusing is active at 10(-10) M or lower in tissue culture.     

Kinetics of CTL induction by concanavalin A.

Induction of maximal CTL activity was achieved within 12 hr of exposure to Con A in vitro in various mouse lymphoid cell populations. These included spleen cells from normal unsensitized mice, spleen cells from mice previously immunized with alloantigen, and mouse spleen cells exposed to alloantigen in long-term mixed leukocyte culture (LTMLC). Although induction of maximal incorporation of tritiated thymidine was accomplished within this same period in the cells obtained from LTMLC, a much longer period of Con A exposure (greater than 24 hr) was required for freshly prepared spleen cells from normal or previously immunized mice. These findings indicate that the increased tritiated thymidine uptake induced in freshly prepared spleen cells on continued exposure to Con A beyond 12 hr is not associated with the development of cytolytic activity, and that it probably represents stimulation of subpopulations no longer present in the LTMLC population where positive selection for cells responsive to cellular alloantigens has taken place.     

Heterogeneity of mouse Thy 1.2 antigen expression revealed by monoclonal antibodies

A different sensitivity of T cells from C57B1/6 and DBA/2 mice to treatment with the monoclonal anti-Thy 1.2 F7D5 serum as compared with a conventional alloantiserum is reported. Depletion of T helper cells, Con A-, PHA-, MLC-, and GVH-reactive cells from a DBA/2 or C57B1/6 spleen cell population was readily achieved with the conventional alloserum. In contrast, the F7D5 antiserum abolished all T functions studied in C57B1/6 spleen cells whereas it was totally or partially ineffective on DBA/2 spleen cells when T helper, MLC, or GVH reactivity were assayed. It did however eliminate the capacity of DBA/2 spleen cells to respond to stimulation with Con A or PHA. Analysis in an Ortho-Cytofluorograf of thymocytes and sIg^? lymphocytes labeled with either GAMB-F or F7D5 + RAM Ig-F showed no difference at the level of the thymocytes: Thy 1.2 antigen as revealed by either GAMB or F7D5 is similarly expressed in the two mouse strains. The fluorescence profiles of splenic T lymphocytes indicated a reduced representation per unit cell basis of the Thy 1.2 antigenic determinant recognized by F7D5 in DBA/2 mice. Moreover, this same determinant is expressed in only 70% of all Thy 1.2-positive cells detected in DBA/2 sIg^? population. This implies that, in DBA/2 mice, maturation of T cells is accompanied by a complete or partial loss of the F7D5 Thy 1.2 determinant and that T helper functions and MLC and GVH reactivity are mediated by T cells which express little or none of this F7D5 Thy 1.2 determinant.     

Thymic lymphocytes. III. Cooperative phenomenon in the proliferation of thymocytes under Con A stimulation

In the present paper, the response of thymocytes to Con A is analyzed in terms of a cooperative phenomenon between medullary thymocytes, cortical thymocytes, thymic accessory cells, and interleukin 2. Medullary thymocytes respond spontaneously to Con A and produce IL-2. The addition of exogenously produced IL-2 enhances their proliferation. Small numbers of cortical (PNA+) thymocytes do not respond to Con A, even in the presence of IL-2-containing supernatant. By increasing the number of PNA+ cells per well, sensitivity to Con A and IL-2 appears. This response may be linked either to the increase in a minor PNA+-responding population and/or to the enhanced contamination by medullary thymocytes and macrophages in non-responding PNA+ thymocyte population. In this hypothesis, either the contaminating cells respond by themselves and/or cooperate with PNA+ cells to induce their proliferation. Coculture of non-responding low numbers of PNA+ thymocytes with Con A- and IL-2-containing supernatant in the presence of PNA- cells containing thymic medullary thymocytes and macrophages always produces a higher response than that of each individual population. These results show that a cooperative phenomenon occurs in the cocultures of PNA+ and PNA- thymic cells. We can show using PNA+ and PNA- thymocytes with different Thy 1 alleles, that indeed both PNA+ and populations participate PNA-thymocytes with different Thy 1 alleles, that indeed both PNA+ and PNA- populations participate in the generation of proliferating cells. We can demonstrate, by lysis experiments with monoclonal antibodies and complement that at the end of coculture, most of the proliferating cells are Lyt 1+, and part are Lyt 2+ or L3T4+. We discuss the fact that the phenotype of the cells after activation does not allow us to deduce the phenotype of their precursors. Lysis of Ia+ cells prior to coculture, reduces the level of the proliferative response but does not modify the percentage of cooperation produced by the coculture. Cooperation with medullary mature thymocytes or the presence of active Ia- accessory cells possibly able to convert to Ia expression during coculture experiments may account for these results.     

Migration of putative progenitor T cells in response to thymus-derived chemotactic factors

Progenitor T cells reach the thymus through the circulation from hematopoietic organs and then migrate toward the site of differentiation in the thymus. The mechanism that regulates such intrathymic migration is not well understood. In order to clarify this mechanism, in vitro chemotactic activity for murine thymocytes was assayed in the extracts and culture supernatants of thymic tissue elements. A potent thymocyte chemotactic activity was found in the extract and culture supernatant from Ig-, Ia- thymic stromal cells. Peanut agglutinin-positive (PNA+1), Thy 1+, TL-, Lyt 1+2-, L3T4- thymocytes, Ig-, Thy 1- bone marrow cells, and mononuclear cells of spleen and peripheral blood, but neither B cells nor lymph node cells, were chemotactically attracted by the factor(s). The chemotactic activity was found in none of the following materials tested: the extract and culture supernatant of thymocytes, culture supernatant of lymph node stromal cells, normal mouse serum, and zymosan-activated serum. The chemotactic activity was found in three molecular fractions by gel chromatography. The activity in all three fractions was destroyed by trypsin digestion or by heating at 56 degrees C for 30 min. These results suggest that Ig-, Ia- thymic stromal cells but not thymocytes secrete a chemotactic factor(s) for progenitor T cells with three molecular species. The factor is considered to play an important role in the migration of intrathymic progenitor T cells into the site of differentiation.     

Strategies of immune reconstitution: effects of lymphokines on murine T cell development in vitro and in vivo

Need for more effective treatment to reconstitute T cell immunity in secondary immunodeficiencies like AIDS prompted an exploration into the roles played by leukocyte products on the ontogeny of murine T lymphocytes in vitro and in vivo. It was observed that mixed lymphokines potently stimulate the proliferation of prothymocytes, immature cortical thymocytes, and mature medullary thymocytes. The effect of the natural, mixed lymphokines could be reproduced in the main by the combination of recombinant interleukin I and II. Mixed lymphokines administered in vivo augmented splenic lymphoproliferative responses in athymic nude mice without T cell marker (Thy 1.2) induction and, in neonatal mice, induced both Thy 1.2 and proliferative responses. Recombinant IL-2 at equivalent dose was less active in nude mice and not active in neonatal mice. The evidence indicates that lymphokines, particularly IL-1 and IL-2 in combination, regulate T cell ontogeny and can act in an endocrine fashion to promote T cell development in T cell-free mice having a functional thymus. Mixed lymphokines may be useful for immune reconstitution in AIDS, however, only if given prior to thymic destruction.     

Induction of allospecific suppressor T cells in vitro.

Incubation of mouse thymic lymphocytes with irradiated allogeneic spleen cells gave rise to suppressor cells. The suppressor activity was assayed by adding the incubated cell mixture to a mixed lymphocyte culture (MLC) in which the responder cells were syngeneic with the sensitized thymocytes and the stimulator cells were syngeneic with the sensitizing spleen cells. Such addition suppressed significantly thymidine incorporation in the mixed lymphocyte reaction (MLR). The suppressor cells were found to carry the θ antigen and to function allospecifically, as shown by cross-testing in three allogeneic combinations. Our data suggest that these cells may originate from immature cortisone-sensitive thymic lymphocytes and also provide some preliminary information concerning their mode of action.     

Biological expressions of lymphocyte activation. IV. Concanavalin A-activated suppressor cells in mouse mixed lymphocyte reactions.

Thymus-derived lymphocytes (T cells) from mouse spleen, activated in vitro or in vivo with concanavalin A (Con A), suppress proliferative responses of syngenic lymphocytes in mixed lymphocyte reactions (MLR). Replication in vitro was not required for expression of suppressor activity by Con A-activated cells and was blocked in MLR by treating suppressor cells with mitomycin C or irradiation. Kinetics of MLR responses and viability of cultures were not altered by addition of activated suppressor cells. The data are consistent with a direct inhibitory effect of suppressor T cells on antigen-induced DNA replication. These observations extend a model previously described for regulation of antibody synthesis by Con A-activated T cells to control of cell-mediated immune responses. This model should be particularly useful in further definition of regulatory T cell subpopulations, and in investigation of interactions and relationships between such populations.     

Characterization of thymic progenitors in adult mouse bone marrow

Thymic cellularity is maintained throughout life by progenitor cells originating in the bone marrow. In this study, we describe adult mouse bone cells that exhibit several features characteristic of prothymocytes. These include 1) rapid thymic engraftment kinetics following i.v. transplantation, 2) dramatic expansion of thymic progeny, and 3) limited production of hemopoietic progeny other than thymocytes. The adult mouse bone marrow population that is depleted of cells expressing any of a panel of lineage-specific Ags, stem cell Ag-1 positive, and not expressing the Thy1.1 Ag (Thy1.1(-)) (Thy1.1(-) progenitors) can repopulate the thymus 9 days more rapidly than can hemopoietic stem cells, a rate of thymic repopulation approaching that observed with transplanted thymocytes. Additionally, Thy1.1(-) progenitors expand prolifically to generate thymocyte progeny comparable in absolute numbers to those observed from parallel hemopoietic stem cell transplants, and provide a source of progenitors that spans multiple waves of thymic seeding. Nevertheless, the Thy1.1(-) population yields relatively few B cells and rare myeloid progeny posttransplant. These observations describe the phenotype of an adult mouse bone marrow population highly enriched for rapidly engrafting, long-term thymocyte progenitors. Furthermore, they note disparity in B and T cell expansion from this lymphoid progenitor population and suggest that it contains the progenitor primarily responsible for seeding the thymus throughout life.     

Antigen expression of mouse spleen × thymocyte heterokaryons

The expression of Thy 1.2 and thymic leukemia (TL) antigens by heterokaryons of spleen cells of strain A mice (A-S) and AKR thymocytes (AKR-T) was determined. The A-S parental cells do not express TL antigens, although strain A thymocytes are TL-positive. Approximately 25% of A-S cells express Thy 1.2 antigens; however, AKR-T cells express a different Thy 1 antigen (Thy 1.1) and are phenotypically negative for TL expression. AKR-T × A-S heterokaryons were prepared with the aid of inactivated Sendai virus. Identification of heterokaryons was facilitated by prior isotopic labeling of AKR-T but not A-S cells, and the finding by autoradiography of binucleated cells with one radioactively labeled and one non-labeled nucleus. Antigenic expression of these fused cells was determined by exposure of the cells to specific antiserum and complement prior to autoradiography. 24 hr after fusion, fused cells were resistant to the cytotoxic effects of TL antiserum and fresh complement. However, a large proportion of these cells was lysed by treatment with antiserum directed against the Thy 1.2 antigen.     

Generation of lymphokine-activated killer (LAK) cells from tumor-infiltrating lymphocytes

Culture of tumor-infiltrating lymphocytes (TIL) containing about 20% BMC2 tumor cells with recombinant human interleukin 2 (rIL-2) resulted in the diminish of tumor cells and the growth of lymphocytes. These IL-2-activated lymphocytes showed a strong cytotoxic activity against not only syngeneic tumor cells but also allogeneic tumor cells. Such broad-reactive killer cells, termed lymphokine-activated killer (LAK) cells, are also inducible from spleen cells by in vitro activation with IL-2. However, LAK cells generated from TIL (TIL-LAK) showed higher cytotoxic activity against BMC2 than LAK cells generated from spleen cells (S-LAK). Furthermore, it was demonstrated that TIL-LAK cells revealed marginal cytotoxic activity against normal Con A blasts and YAC-1 cells as opposed to S-LAK. Flow cytometric analysis of TIL-LAK indicated that TIL-LAK cells mainly consisted of Thy 1.2+, Ly 2+, asialo GM1+ cells. TIL-LAK cells displayed not only in vitro cytotoxicity but also in vivo anti-tumor activity. Furthermore, it was also confirmed that TIL-LAK cells could be induced in autochthonous mouse tumor systems and human gastric tumor systems.     

Production of thymocyte-stimulating factor by murine spleen lymphocytes: cellular and biochemical mechanisms.

Cultures of murine spleen lymphocytes treated with Thy 1.2 antiserum plus complement do not produce thymocyte-stimulating factor (TSF). The population of thymocytes composed of immunocompetent, low-density cells produces only small amounts of TSF. Experiments with cyclophosphamide-injected mice and with spleen cells treated in vitro with antiserum to the murine B lymphocyte antigen plus complement and experiments using spleen cells stimulated in vitro with Sepharose-bound phytohemagglutinin indicate that B lymphocytes neither cooperate with T lymphocytes for the production of TSF nor produce TSF. Some lectins (pokeweed mitogen, Lens culinaris hemagglutinins A and B) have been found to induce the production of TSF by spleen cells. Other lectins (wheat germ agglutinin, Agaricus bisporus agglutinin) and sodium periodate do not. Spleen cells of mice immunized in vivo with keyhole limpet hemocyanin bound to bentonite particles or with BCG produce TSF when challenged in vitro with the specific antigen. Experiments using inhibitors of the macromolecular metabolism showed that DNA synthesis is not required for the production of TSF by spleen lymphocytes, whereas RNA and protein synthesis are required. Resolution of spleen lymphocytes on a discontinuous albumin gradient into six subpopulations showed that the TSF activity was rather uniformly distributed among the various subpopulations of cells.     

Thymocyte-stimulating factor from peripheral blood cells.

Human peripheral blood leukocytes (HPBL) produce a thymocyte-stimulating factor (TSF-HPBL) that enhances the phytohemagglutinin (PHA) and concanavalin A (Con A) responsiveness of murine thymocytes. This activity is considerably specific for thymocytes. TSF-HPBL is not mitogenic by itself. Experiments with cell cultures pretreated with carbonyl iron particles showed that phagocytic cells are not involved in the production of mouse and rat TSF but are involved in the production of TSF-HPBL. The dose-response profile to PHA of murine thymocytes cultured in the presence of TSF-containing supernatants is similar to that of mature, immunocompetent spleen cells. TSF-HPBL, however, does not enhance the PHA responsiveness of murine thymocytes at low (<0.25 μg/microwell) concentrations of mitogen. TSF enhances the PHA and Con A responsiveness of the high-density subpopulations of thymocytes isolated on a Ficoll-Hypaque gradient. In general, the enhancing effect of TSF-HPBL on these subpopulations of thymocytes is smaller than that exerted by TSF. While supernatants containing TSF confer to thymocytes the ability to participate in a mixed lymphocyte reaction, this effect is not exerted by supernatants containing TSF-HPBL. A factor enhancing the PHA and Con A responsiveness of murine thymocytes is also produced by murine peripheral blood leukocytes (TSF-MPBL). This factor, similarly to TSF-HPBL, is produced by phagocytic cells and does not confer to murine thymocytes the ability to participate in a mixed lymphocyte reaction. Human T-cell lines do not enhance the PHA or Con A responsiveness of murine thymocytes. TSF-HPBL has a molecular weight of about 30,000 daltons, as measured by Sephadex filtration. Its half-time of inactivation as 56 °C is 162 ± 8 min.     

In vivo generation of suppressor T cells by thymosin in congenitally athymic nude mice

Treatment of nude mice with thymic factors such as thymosin has been mostly ineffective in generating effector T cells. This study examined the effects of treating nude mice with thymosin fraction 5 on the induction of cells that could participate in and/or regulate cytotoxic T lymphocyte (CTL) generation by normal spleen cells in vitro. Splenic lymphocytes from BALB/c nude mice injected with thymosin fraction 5 every other day for 2 wk were tested for their ability to generate CTL in vitro. Two days after the last subcutaneous injection of thymosin, nude spleens were removed, mixed with normal BALB/c spleen cells, and placed into a mixed lymphocyte tumor culture (MLTC) against allogeneic RBL 5 tumor cells. After a 5-day incubation, cultures were tested for the presence of CTL in a 4-hr 51Cr-release assay. Spleen cells from thymosin-treated nude mice did not generate CTL but suppressed the ability of normal spleen cells to generate CTL in vitro. Characterization of the thymosin-induced nude mouse suppressor cells showed them to be Thy 1 positive, nonadherent, cyclophosphamide-sensitive T cells. These data demonstrate that some T cell maturation occurs in vivo under thymosin influence. However, the activity of these cells is initially limited to a regulatory function. These studies suggest that maturation of functional suppressor T cells occurs before CTL. Further immunologic manipulation appears to be necessary in order to induce CTL effector cells in nude mice.     

A chemotactic factor for rat thymocytes may regulate T-lymphocyte migration toward the thymic microenvironment

Using a modified Boyden chamber assay, extracts or culture supernatants of rat thymic stromal cells, or thymocytes were examined by chemotactic activity to rat leukocytes. Rat thymocytes responded chemotactically to the aqueous extract as well as to culture supernatants of thymic stromal cells. However, neither the extract and culture medium from concanavalin A-stimulated thymocytes nor any component of rat serum has shown such an activity. The thymic extract was fractionated into three molecular species with chemotactic activity for thymocytes. The thymocyte chemotactic factor(s) (TCFs) in the extract was distinct from known lymphocytic chemotactic factors, such as interleukin-1 (IL-1), IL-2, C5a, and the culture supernatant of stimulated thymocytes. In vitro, TCFs could attract, in addition to thymocytes, bone marrow cells, fetal liver cells, and nylon-wool nonadherent lymphocytes from peripheral blood and spleen. Lymph node cells, neutrophils, macrophages, and B cells from peripheral blood could not respond to TCFs. Thymocytes also responded to the extract of splenic stromal cells. Unlike the thymic extract, however, the splenic extract was chemotactically active for lymphocytes from lymph nodes but not for bone marrow cells. These results indicate that thymic stromal cells secrete a chemotactic factor(s) for a relatively immature type of T-lineage cells, which may by a thymus-homing progenitor T cell, while spleen may contain an attractant for a relatively mature type of T-lineage cells.     

Effects of thymic myoid cell culture supernatant on cells from lymphatic tissues

Conditioned media (MCM) of cloned thymic myoid cells (IT45R92, R613Ad, and R615B2) were used to investigate their possible involvement in thymic biological events. Those myoid cells produced in a culture medium biological activities capable of stimulating the growth of thymocytes, spleen cells, and bone marrow cells of mice and rats. Surface markers detected on spleen cells proliferating in MCM were characteristic of monocyte-macrophage lineages (C3R, Fc gamma R, asialo GM1) and T-cell lineages (Thy 1) but not B cells (sIgG). Chromatographic studies also suggested that the biological activities of MCM could be separated into two different molecular entities, such as a colony-stimulating activity and an interleukin 1-like activity which supported the growth of monocyte-macrophage lineages and T-cell lineages, respectively. These results indicate that thymic myoid cells produce cytokines important for the regulation of intrathymic interleukin cascade by which clonally differentiated thymic lymphocytes may be expanded into a sizable pool.     

The role of cellular proliferation in the induction of experimental autoimmune thyroiditis (EAT) in mice

Experimental autoimmune thyroiditis (EAT) can be induced in susceptible strains of mice by injection of mouse thyroglobulin (MTg) and adjuvant. Lymphocytes from immunized mice develop a proliferative response to MTg which generally correlates with the development of EAT. We utilize a cell transfer system wherein spleen cells from CBA/J mice primed with MTg and lipopolysaccharide (LPS) in vivo are activated by culture with MTg in vitro to transfer EAT to naive recipients. In vivo priming of CBA/J mice is required to develop an antigen specific proliferative response to MTg. This response is optimal between 48 and 90 hr of culture at an MTg concentration of 125-250 micrograms/ml. The correlation between proliferation and transfer of EAT is not absolute as primed Balb/c X CBA/J F1 and AKR lymphocytes do not proliferate detectably in response to MTg but can be activated to transfer EAT; primed Balb/c lymphocytes neither proliferate nor transfer EAT. Proliferation per se is not sufficient to activate cells to transfer EAT as culture with nonspecific mitogens is not effective in activating primed CBA/J spleen cells to transfer EAT. However, lymphoblasts generated during in vitro culture of primed CBA/J spleen cells with MTg are responsible for transfer of EAT; small lymphocytes are ineffective. We conclude that antigen specific proliferation in response to MTg is essential in activating lymphocytes in vitro to transfer EAT.