Experimental induction and termination of acute psychological stress in human volunteers: effects on immunological, neuroendocrine, cardiovascular, and psychological parameters.

The present research investigated the effects of controlled experimental manipulations of stress on biological and psychological reactions. Fifty young adult male volunteers were exposed to a 12-min period of stress induced by the threat of an unavoidable, painful electric shock. A 12-min period without this threat preceded or followed the stress period. Blood was drawn during the 4th and the 12th minute of each period. Anticipatory threat led to significant elevations in the proportions and cytotoxic activity of natural killer (NK) lymphocytes, plasma epinephrine levels, pulse rate, and reported level of tension, and to a reduction in the CD4/CD8 ratios. The no-threat period induced a return to baseline values for epinephrine, pulse rate, and tension, and lower than baseline levels for cytotoxic activity of NK lymphocytes, within a similarly short time span. The findings underline the rapidity with which physiological changes may transpire in the course of a brief and acute period of psychological stress, and the rapidity of their reversal upon relief from the stressor.

Prenatal stress depresses immune function in rats.

The aim of the present study was to determine the effect of prenatal stress on immune function in rats. Pregnant rats were stressed by noise and light, three times weekly throughout pregnancy. Experiments were performed on male and female offspring aged 2 months. Cellular immune responses of splenic lymphocytes to B-cell (pokeweed mitogen (PWM) and T-cell (phytohemagglutinin (PHA)) mitogens were measured by [3H]thymidine uptake, and natural killer (NK) cell cytotoxicity in blood and splenic lymphocytes was measured against the murine T-cell lymphoma, YAC-1, by a 4-h [51Cr] release assay. Prenatal stress suppressed immune function as shown by a) decreased NK cytotoxicity in splenic and blood lymphocytes, indicating that the effect was not confined to a particular immune compartment, and b) decreased rate of proliferation of splenic lymphocytes to PWM and a smaller depressant effect on their response to PHA. The suppression of B-cell proliferation was more marked in the female and that of NK cell cytoxicity, in the male. Prenatal stress did not alter the distribution of subsets of lymphocytes, in either the spleen or blood, indicating that the reduction in proliferative and cytotoxic activity resulted from functional modifications of effector mechanisms in the cells rather from alterations in their migration between immune compartments. The mechanisms underlying this effect of prenatal stress are not clear but could result from an action of maternal stress hormones on the developing fetal neuroendocrine system.

Studies in psychoneuroimmunology: psychological, immunological, and neuroendocrinological parameters in Israeli civilians during and after a period of Scud missile attacks.

Twenty-two male volunteers in Jerusalem were subjected to a battery of psychological tests at the height of the Iraqi Scud missile attacks on Israeli cities during the 1991 Persian Gulf War and again after the cessation of hostilities. Venous blood samples were taken at each time point. The separated mononuclear cells and plasma were cryopreserved, and a spectrum of immunological and neuroendocrine assays were performed on the preserved samples. Psychological testing indicated levels of anxiety were higher during the war than they were after the war ended, and both anxiety and anger during the hostilities were significantly elevated in comparison with prewar data. During the war, specific war-related pressures were greater than everyday pressures, and problem-focused coping was more evident than emotion-focused coping. Natural-killer cell activity and cell-mediated lympholysis were significantly elevated during the war, as were plasma levels of adrenocorticotrophic hormone, neurotensin, and substance P. The only biological test parameter found to be reduced during the war period was mononuclear cell thymidine incorporated in nonstimulated cultures.

Restraint stress-induced changes in lymphocyte subsets and the expression of adhesion molecules.

The effect of restraint stress on the distribution of lymphocyte subsets were studied in young BALB/c male mice. Loss of whole body weight, a reduction in the weights of spleen and lymph nodes, and higher levels of serum corticosterone were evident after a single continuous restraint period of 16-18 h. Tissue sections of spleens from restrained animals revealed erythrocyte depletion in the contracted red pulp. Furthermore, there was a significantly higher proportion of CD4+, but not of CD8+, lymphocytes in the spleen. The proportion of the CD4+ subset was markedly diminished in peripheral blood, whereas no changes were detected in lymph nodes. Restraint resulted in enhanced allogeneic mixed lymphocyte reactivity and in altered expression of some CD4+, but not CD8+, splenocyte adhesion molecules (CD44, LFA-1 and VLA-4). Removal of circulating corticosteroids by surgical adrenalectomy abolished the restraint-induced changes in lymphocyte adhesion molecule expression. The findings suggest that the observed differences in lymphocytes subset distribution of lymphoid organs may be due to changes in the pattern of adhesion molecule expression.

A role for CdxA in gut closure and intestinal epithelia differentiation.

CdxA is a homeobox gene of the caudal type that was previously shown to be expressed in the endoderm-derived gut epithelium during early embryogenesis. Expression of the CDXA protein was studied during intestine morphogenesis from stage 11 (13 somites) to adulthood in the chicken. The CDXA protein can be detected during all stages of gut closure, from stage 11 to 5 days of incubation, and is mainly localized to the intestinal portals, the region where the splanchnopleure is undergoing closure. In this region, which represents the transition between the open and closed gut, the CDXA protein is restricted to the endoderm-derived epithelium. At about day 5 of incubation, the process of formation of the previllous ridges begins, which marks the beginning of the morphogenesis of the villi. From this stage to day 11 expression of CDXA is localized to the epithelial lining of the intestine. In parallel, a gradual increase in CDXA protein expression begins in the mesenchyme that is close in proximity to the CDXA-positive endoderm. Maximal CDXA levels in the mesenchyme are observed at day 9 of incubation. During days 10 and 11 CDXA levels in the mesenchyme remain constant, and by day 12 CDXA becomes undetectable in these cells and the epithelium again becomes the main site of expression. From day 12 of incubation until adulthood the CDXA protein is present in the intestinal epithelium. Until day 18 of incubation expression can be detected along the whole length of the villus with a stronger signal at the tip. With hatching the distribution along the villi changes so that the main site of CDXA protein expression is at the base of the villi and in the crypts. The transient expression of CDXA in the mesenchyme between days 5 and 11 may be related to the interactions taking place between the mesenchyme and the epithelium that ultimately result in the axial specification of the alimentary canal and the differentiation of its various epithelia. The main CDXA spatial distribution during morphogenesis suggests a tight linkage to the formation and differentiation of the intestinal epithelium itself. CDXA appears to play a role in the morphogenetic events leading to closure of the alimentary canal. During previllous ridge formation the CDXA protein is transiently expressed in the mesenchymal cells thought to provide instructive interactions for the regionalization and differentiation of the gut epithelium.(ABSTRACT TRUNCATED AT 400 WORDS)

The chicken CdxA homeobox gene and axial positioning during gastrulation.

The chicken homebox containing gene, CdxA (formerly CHox-cad), was previously shown to be expressed during gastrulation. Localization of CdxA transcripts by in situ hybridization to tissue sections revealed that, during gastrulation, expression of this gene exhibits a posterior localization along the primitive streak. The transcripts are localized to epiblast cells in the vicinity of the primitive streak, to cells of the primitive streak itself and in the definitive endoderm as it replaces the hypoblast. In order to study in greater detail the pattern of expression of the CdxA gene during gastrulation, we expressed the full-length CdxA protein as a fusion protein in E. coli and generated monoclonal antibodies against it. Chicken embryos at different stages of gastrulation were processed for whole-mount immunohistochemical localization of the protein using anti-CdxA antibodies. Once the pattern of expression in the whole embryo was determined, the same embryos were sectioned to determine the identity of the cells expressing the CdxA protein. Detailed analysis of the CdxA protein in embryos, from the onset of primitive streak formation to the beginning of the tail bud stage (stages 2 to 10), has shown different patterns of expression during primitive streak elongation and regression. The CdxA protein is initially detected at the posterior marginal zone and the expression moves rostrally into the primitive streak during mid-streak stages. As the primitive streak elongates, the CdxA stripe of expression moves anteriorly. By definitive streak stages, the CdxA stripe of expression delineates a position along the anterior-posterior axis in the primitive streak. CdxA, like its Drosophila homologue cad, is expressed during gastrulation in a stripe localized to the posterior region of the embryo. These observations suggest that CdxA as a homebox gene may be part of a regulatory network coupled to axial determination during gastrulation in the early chick embryo.

Regulatory interactions among autologous T cell clones. Human bifunctional T cell clones regulate the activity of an autologous T cell clone.

In contrast to the ease of cloning and characterizing, at the molecular level, helper and cytotoxic T cells, suppressor T cells remain an enigma, and their existence as discrete entities is being increasingly challenged. Here we review evidence that CD4+ regulatory clones, capable of expressing both helper and suppressor functions, may account for much of the suppressor function. It is suggested that a single T cell clone, depending on the signals it receives from its environment, may release either helper or suppressor cytokines. Studying such clones under defined conditions (providing suppressor signals), may preclude detection of their helper capacity. Since some therapeutic approaches in various human diseases are based on the manipulation of helper and suppressor functions, the question whether committed suppressor cells exist has important practical implications in medicine.

Specific interactions between a human CD4+ clone and autologous CD4+ bifunctional immunoregulatory clones.

The cellular communications between a human CD4+ clone and autologous CD4+ clones induced with the first clone are described. The autoreactive clones proliferated after stimulation with the inducer clone, but not after stimulation with autologous clones expressing irrelevant specificities. The inducer clone markedly lost its ability to interact with the autoreactive clones after the modulation of its T-cell receptor. The proliferation of the autoreactive clones stimulated with the inducer clone was blocked by anti-DR monoclonal antibody. Collectively, these findings indicate that the autoreactive clones recognize idiotypic-like determinants on the receptor of the inducer clone in conjunction with DR antigen. The regulatory activity of the autoreactive clones was assayed by co-cultivation with their target inducer clone. The autoreactive clones were not committed to a single program, they could either suppress or enhance the proliferation of the target cells depending on the state of activation of the target cells. Activated target cells were suppressed whereas non-activated cells were enhanced. It is predicted that antagonistic cytokines released from the autoreactive clones exert differential effects on the target clone.

Interactions between autologous T cell clones.

A human CD4 clone (Mx9/9) using the V beta 8 receptor was used as antigen to generate autologous clones (termed anti-Mx9/9 clones) which proliferate in response to this clone, but not other autologous clones. This was used as an experimental model to explore the specific interactions between autologous T cells. Anti-HLA-DR monoclonal antibodies inhibited the response of the anti-Mx9/9 clones, suggesting that these clones recognize their target antigen in association with HLA-DR. Because of the specificity of the anti-Mx9/9 clones for the initiating clone (Mx9/9), but not any other autologous V beta 8- or V beta 8+ CD4 clones, the target antigen seems to be part of the T cell receptor, but not V beta 8 itself. However, the anti-Mx9/9 clones responded also to the autologous EBV line, and thus the target antigen is not known. The regulatory activity of the anti-Mx9/9 clones was assayed by coculture with their target clone. A variety of responses were seen, both inhibitory and stimulatory, which varied depending on the "conditions" of the T cell used. These results suggest that T cells interact in a complex network, perhaps as complex as the regulatory interactions between antibody molecules and B cells.

Auto-delayed-type hypersensitivity induced in immunodeficient mice with modified self-antigens. V. Cellular autoreactivity directed against self-H-2Dd subregion mediates the inflammatory responses.

X-irradiated (250 rads) A mice injected with syngeneic trinitrophenylated spleen cells (Syn-TNP-SC) developed syngeneic delayed-type hypersensitivity (Syn-DTH) after footpad challenge with syngeneic concanavalin A-induced lymphoblasts (Syn-Con A blasts), as indicated by footpad swelling and lymph-node proliferation assays. Genetic analysis with different recombinant strains of mouse revealed that the H-2Dd subregion restricts these immunological activities. A small immunogenic entity was isolated from Syn-TNP-SC extract by gel filtration and affinity chromatography with anti-Dd antibody. X-irradiated A mice injected with this antigen also generated inflammatory responses after challenge with Syn-Con A blasts. Analysis of the data obtained from the gel filtration and the affinity chromatography suggest that the Syn-DTH-stimulating antigen is a fragment of the H-2Dd heavy chain with a molecular weight (MW) of 5,000 to 10,000. This fragment does not contain TNP molecules or sugar moieties and neither does it express affinity for antibodies against class II products. Ltk-cells transfected with the H-2Dd gene also induced Syn-DTH in X-irradiated A mice, while the parent cells failed to stimulate such a response. Taken together these findings indicate that the selection process in the thymus is not absolute and T cells recognizing self-major histocompatibility complex (MHC) products do migrate from this organ to the periphery of the immune system. The possible biological significance of these autoreactive cells is discussed.

Anti-interleukin 2 receptor antibody suppresses delayed-type hypersensitivity to foreign and syngeneic antigens.

Delayed-type hypersensitivity (DTH) requires stimulation of antigen-specific helper T cells (Th). Because de novo expression of the interleukin 2 receptor (IL 2R) is a necessary step in T cell activation, we tested the capacity of anti-mouse IL 2R monoclonal antibody (Mab) and anti-Th Mab (anti-L3T4) to block DTH. We examined the effect of these Mab on two distinct DTH systems, i.e., to foreign hapten (trinitrobenzenesulfonic acid) and to this hapten present on syngeneic blasts. Both anti-IL 2R and anti-L3T4 Mab suppress DTH. Therapy is as effective treating with one injection just before challenge with the hapten as giving six daily injections. These data indicate that DTH is dependent on a discrete subset of activated IL 2R-positive T cells, because anti-IL 2R therapy, which targets few cells, is as effective as anti-L3T4 Mab treatment, which targets the entire Th subset.

Auto-delayed-type hypersensitivity induced in immunodeficient mice with modified self-antigens. III. Suppressive T-cell factor controls the autoreactivity against self-antigens.

X-irradiated (250 rad), cyclophosphamide-treated or ATx A mice injected with syngeneic trinitrophenylated spleen cells (TNP-SC) and footpad challenged with syngeneic lymphoblasts generated delayed-type hypersensitivity (DTH) responses 24, 48 and 72 h after challenge. The syngeneic-DTH (syn-DTH) response was mediated by Lyt-1+ cells and suppressed with Lyt-1+2+3+, I-Jk+ cells. The suppressor cells were obtained from spleens or thymuses of normal syngeneic mice. Suppressor factor (SF) was extracted or released from Lyt-1+2+3+, I-Jk+ cells obtained from normal A mice (but not from X-irradiated A mice). The factor blocked the DTH responses of X-irradiated mice injected with syngeneic TNP-SC and challenged with syngeneic lymphoblasts when injected into the mice both at the induction phase and the elicitation phase of the DTH. The factor failed to abrogate allogeneic and xenogeneic DTH. However, allogeneic factor (derived from C57BL/6 mice) abolished the syn-DTH response of mice injected with syngeneic TNP-SC and challenged with syngeneic lymphoblasts. The SF was produced by Lyt-1+2+3+, I-Jk+ T cells or by thymocytes. The combined extracted product of Lyt-1+ and Lyt-2+ cells did not abrogate the syn-DTH response. Normal spleen cells depleted of phagocytes by a magnetic procedure also produced the SF. These findings indicate, therefore, that suppressive factor (or factors; see Discussion in the accompanying paper, Ref. 17) controls the immunological autoreactivity against syngeneic TNP-SC.

Auto-delayed-type hypersensitivity induced in immunodeficient mice with modified self-antigens. IV. Characterization of the suppressive T-cell factor that controls the autoreactivity against self-antigens.

Suppressor cells obtained from spleens of normal A mice, or factor extracted from these suppressor cells, abolished the syngeneic delayed-type hypersensitivity (syn-DTH) response of X-irradiated A mice injected with trinitrophenylated spleen cells and challenged with syngeneic lymphoblasts. Some of the physical, chemical and biological properties of the suppressive factor (SF) were characterized. The SF was relatively temperature-stable and its activity was destroyed by pronase (but not with RNase or DNase). The activity of the SF was absorbed on concanavalin A and anti-I-Jk Sepharose columns, suggesting that the factor is a glycoprotein-bearing I-Jk product. The approximate molecular weight of the factor is 50,000-60,000. The SF was absorbed on plastic adherent cells (but not on non-adherent cells). Adherent cells that absorbed the SF abrogated the ability of primed T cells to transfer the syn-DTH to naive X-irradiated recipients. In contrast, SF that was presented directly to the primed T cells failed to abolish their ability to transfer DTH. These findings suggest that the adherent cells serve as mediators, transferring the SF from factor-producing cells (Lyt-1+2+3+, I-Jk+ T cells) to target cells (Lyt-1+ primed T cells).

Auto-delayed-type hypersensitivity induced in immunodeficient mice with syngeneic modified self-antigens. II. Suppressor T cells control the autoimmune response.

The control of the autoimmune response to modified self-antigens was explored, using immunodeficient mice injected with syngeneic trinitrophenylated spleen cells (TNP-SC) as an experimental model system. X-irradiated (250 rad) A mice injected with TNP-SC and footpad-challenged 7 to 14 days later with syngeneic lymphoblasts generated a delayed-type hypersensitivity (DTH) response that was expressed by footpad swelling measured 24 h, 48 h and 72 h later. Histopathological examination showed massive inflammatory infiltration in the soft tissues of the limbs with extensive necrosis. This was not observed in X-irradiated mice that received the lymphoblast challenge only. The immunological activity was transferred from the X-irradiated TNP-SC-immunized mice to naive recipients by T cells (Lyt-1+) and not by serum, thus excluding the possibility that the inflammatory reaction is mediated by antibodies. We have previously presented evidence that the differentiation status of the lymphoblasts, and not contaminants from the incubation media, was the determinant factor eliciting the DTH response of immunodeficient mice injected with TNP-SC. Since only syngeneic lymphoblasts were able to elicit the DTH response of immunodeficient mice injected with syngeneic TNP-SC, we suggested that immunological activity was directed against self-antigens, thus expressing an autoimmune reactivity. The ability of immunodeficient mice to generate syngeneic DTH was not restricted to the TNP hapten or to inbred A-strain mice. X-irradiated BALB/c mice injected with syngeneic penicillinated spleen cells and challenged with syngeneic lymphoblasts generated a significant DTH response, in contrast to X-irradiated BALB/c mice exposed to the challenge dose only. X-irradiated A mice injected with syngeneic TNP-SC and simultaneously reconstituted with syngeneic splenocytes failed to generate a DTH response after the lymphoblast challenge, indicating that the syngeneic DTH response is controlled by normal suppressor cells. The suppressor cells were characterized as T cells carrying I-Jk, Lyt-1+, Lyt-2+ and Lyt-3+ antigenic markers. The suppressor cells abrogated the syngeneic DTH response of immunodeficient mice injected with TNP-SC, even when transferred a few days after the induction of immunological activity, but not when transferred 1 h before the lymphoblast challenge, indicating that even the established immunological activity can be restrained. Various immunological aspects of these observations and the significance of the findings in illuminating human autoimmune disorders are considered.

Delayed-type hypersensitivity induced in immunodeficient mice with syngeneic modified self antigens: a suggestive model of autoimmune response.

Previous studies suggested that trinitrophenyl (TNP)-modified syngeneic red cells induced humoral autoimmune response in mice with defective T cell function but not in normal mice. The ability of modified self antigen to induce autoimmune response in immunodeficient mice was further explored using the delayed-type hypersensitivity (DTH) as an assay system. Mice were immunized with syngeneic TNP-modified spleen cells (TNP-SC) and challenged by syngeneic nonmodified concanavalin A (Con A) or lipopolysaccharide (LPS)-stimulated spleen cells injected into their footpads. The DTH response was assessed 24, 48 and 72 h later by measuring the footpad swelling and was transferred to naive recipients with enriched T cells from TNP-SC-immunized irradiated A mice but not with serum or non-T cells. Adult thymectomized, X-irradiated (250 rds) and cyclophosphamide-treated mice injected with syngeneic TNP-SC generated a DTH response when subsequently challenged with syngeneic lymphoblasts (induced with Con A or LPS) but not when challenged with allogeneic blast cells. In contrast, normal mice treated in a similar manner exhibited a much less significant DTH response. SC incubated 1 to 3 h with Con A failed to elicit the DTH response of immunodeficient mice previously injected with TNP-SC. Both lymphoblasts that were induced in vitro with Con A diluted in fetal calf serum or in normal mouse serum-containing media, and lymphoblasts that were induced in vivo by interleukin 2 elicited DTH responses in X-irradiated, TNP-SC immunized mice. The syngeneic DTH response of the immunodeficient mice injected with TNP-SC was abrogated when they were simultaneously transplanted with syngeneic SC or nylon wool-passed syngeneic SC. If the transplanted splenocytes had been treated with anti-Thy-1 antiserum and complement they failed to abrogate the syngeneic-DTH response of the above mentioned mice. This result suggests that suppressor cells are programmed to control the autoimmune response induced with modified self antigens.

Induction of antitumor reactive cells or suppressor cells by different molecular species isolated from the same nonimmunogenic tumor.

The inability of spontaneous and some laboratory-induced tumors to stimulate the immune system has continuously raised the question of the validity of using immunological maneuvers in order to control tumor growth. In this project we suggest that a tumor which is nonimmunogenic still has an immunogenic potential that can be revealed and used in order to stimulate antitumor immunity and consequently tumor destruction. YAC, a Moloney-virus-induced tumor of A mice, failed to stimulate immunological responses. This tumor homogenate was exposed to nonreduced sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). At the end of the electrophoresis, the gels were sliced and injected in sequential order into various groups of A mice. It was found that some of the gel slices (usually with M.W. of 100 K or less) induced cytotoxic responses, whereas other gel slices (usually with M.W. of about 150 K) induced suppressor cells. Similarly, certain gel slices induced cells that inhibited the in vivo tumor growth, whereas others enhanced the in vivo tumor growth. These last two types of cells did not present the same cellular population that mediated the cellular cytotoxicity or the suppressive effects respectively. It was concluded that poorly immunogenic tumor cells do possess immunogenic potential that can be revealed after dissociation between the immunogenic and suppressogenic entities.

Control of autoimmune responses induced with modified self antigens.

Our previous studies suggested the TNP-modified syngeneic red blood cells induced a humoral autoimmune response in mice with a defective T cell function but not in normal mice. This report describes the continuation of these studies in which we elucidated the auto-delayed-type hypersensitivity response of mice with a defective T cell function. The mice were immunized with syngeneic TNP-modified spleen cells and challenged 14 d later by injecting syngeneic nonmodified con A-stimulated spleen cells into the footpad. The DTH response was assessed 24 hr later. Adult thymectomized, x-irradiated (250 rad), and Cy-treated mice injected with syngeneic TNP-Sc generated a DTH response when subsequently challenged with syngeneic blast cells but not when challenged with allogeneic blast cells. In contrast, normal mice treated in a similar manner did not exhibit a significant DTH response. The auto-DTH response of x-irradiated mice injected with TNP-Sc was abrogated when they were simultaneously transplanted with syngeneic spleen cells or nylon wool-passed syngeneic spleen cells. If the transplanted splenocytes had been treated with anti-Thy 1 serum and complement, they failed to abrogate the auto-DTH response of the above-mentioned mice. These results suggest that suppressor cells are programmed to control the autoimmune response induced with modified self antigens.

Immunogenicity of subcellular fractions and molecular species of MuLV-induced tumors. III. Stimulation of syngeneic antitumor responses by subcellular fractions and molecular species of Moloney virus-induced tumors in CBA and A mice.

YBA, a Moloney virus-induced leukemia in CBA mice, and a relatively weak immunogenic tumor, was screened for the presence of immunogenic antigens. The tumor was subjected to homogenization and subcellular fractionation on sucrose gradients; the immunogenic subcellular fractions underwent further separation by sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE). The immunogenicity of the subcellular fractions and the SDS-PAGE-isolated molecular species were tested by (their) subcutaneous injection into syngeneic mice and examination of their splenocytes examined against tumor cell and normal cell targets by the chromium release cell-mediated lympholysis assay. Tumor cell homogenates were also separated by SDS-PAGE and tested for immunogenicity without prior fractionation. Splenocytes from mice that had received injections of certain SDS-PAGE-isolated epitopes derived from YBA tumor homogenate or its light and heavy subcellular fractions generated effective cytotoxic responses against YBA target cells after 6 days in vitro cultivation. In contrast, intact YBA tumor cells or non-separated tumor homogenates failed to induce an efficient cytotoxic response. The effector cells induced with the immunogenic SDS-PAGE-isolated epitopes of YBA tumor were specific, since they cytolysed the homologous target cells more efficiently than unrelated target cells or syngeneic normal cells. The activity of these effector cells was affected by varying the effector: target ratio. Augmentation of the cytotoxic responses was obtained when the splenocytes of mice immunized with SDS-PAGE-isolated epitopes of YBA tumor were restimulated in vitro, with the homologous neoplastic cells. Immunogenic SDS-PAGE epitopes were isolated from YAC tumor also (YAC is a Moloney-induced tumor of A mice). The effector cells induced with these separated epitopes were characterized as thymus-derived cells and not as natural killer cells. The results suggest that (1) the molecular repertoire of YBA and YBA tumors contain immunogens that can induce a specific antitumor cell-mediated response; (2) the isolated molecular species injected are more efficient immunogens than the entire, unseparated homogenate sample or a dose of 10(8) intact inactivated tumor cells; and (3) the gel matrix may be responsible for the enhanced cell-mediated response induced against the weakly immunogenic tumor.