Complexity of the state of immunological tolerance

Antileukemia and antitumor effects of the graft-versus-host disease: a new immunovirological approach

In leukemic mice, the native host's explicit and well-defined immune reactions to the leukemia virus (a strong exogenous antigen) and to leukemia cells (pretending in their native hosts to be protected "self" elements) are extinguished and replaced in GvHD (graft-versus-host disease) by those of the immunocompetent donor cells. In many cases, the GvHD-inducer donors display genetically encoded resistance to the leukemia virus. In human patients only antileukemia and anti-tumor cell immune reactions are mobilized; thus, patients are deprived of immune reactions to a strong exogenous antigen (the elusive human leukemia-sarcoma retroviruses). The innate and adaptive immune systems of mice have to sustain the immunosuppressive effects of leukemia-inducing retroviruses. Human patients due to the lack of leukemiainducing retroviral pathogens (if they exist, they have not as yet been discovered), escape such immunological downgrading. After studying leukemogenic retroviruses in murine and feline (and other mammalian) hosts, it is very difficult to dismiss retroviral etiology for human leukemias and sarcomas. Since no characterized and thus recognized leukemogenic-sarcomagenic retroviral agents are being isolated from the vast majority of human leukemias-sarcomas, the treatment for these conditions in mice and in human patients vastly differ. It is immunological and biological modalities (alpha interferons; vaccines; adoptive lymphocyte therapy) that dominate the treatment of murine leukemias, whereas combination chemotherapy remains the main remission-inducing agent in human leukemias-lymphomas and sarcomas (as humanized monoclonal antibodies and immunotoxins move in). Yet, in this apparently different backgrounds in Mus and Homo, GvHD, as a treatment modality, appears to work well in both hosts, by replacing the hosts' anti-leukemia and anti-tumor immune faculties with those of the donor. The clinical application of GvHD in the treatment of human leukemias-lymphomas and malignant solid tumors remains a force worthy of pursuit, refinement and strengthening. Graft engineering and modifications of the inner immunological environment of the recipient host by the activation or administration of tumor memory T cells, selected Treg cells and natural killer (NKT) cell classes and cytokines, and the improved pharmacotherapy of GvHD without reducing its antitumor efficacy, will raise the value of GvHD to the higher ranks of the effective antitumor immunotherapeutical measures. Clinical interventions of HCT/HSCT (hematopoietic cell/stem cell transplants) are now applicable to an extended spectrum of malignant diseases in human patients, being available to elderly patients, who receive non-myeloablative conditioning, are re-enforced by post-transplant donor lymphocyte (NK cell and immune T cell) infusions and post-transplant vaccinations, and the donor cells may derive from engineered grafts, or from cord blood with reduced GvHD, but increased GvL/GvT-inducing capabilities (graft-versus leukemia/tumor). Post-transplant T cell transfusions are possible only if selected leukemia antigen-specific T cell clones are available. In verbatim quotation: "Ultimately, advances in separation of GvT from GvHD will further enhance the potential of allogeneic HCT as a curative treatment for hematological malignancies" (Rezvani, A.R. and Storb, R.F., Journal of Autoimmunity 30:172-179, 2008 (see in the text)). It may be added: for cure, a combination of the GvL/T effects with new targeted therapeutic modalities, as elaborated on in this article, will be necessary.

Targeting the proteome/epitome, implementation of subtractive immunization

Monoclonal antibody technology has generated invaluable tools for both the analytical and clinical sciences. However, standard immunization approaches frequently fail to provide monoclonal antibodies with the desired specificity. Subtractive immunization provides a powerful alternative to standard immunization and allows for the production of truly unique antibodies. With the intent of targeting specific epitopes within the proteome, subtractive immunization has been broadly and successfully implemented for the production of monoclonal antibodies otherwise unobtainable by standard immunization. Subtractive immunization utilizes a distinct immune tolerization approach that can substantially enhance the generation of monoclonal antibodies to desired antigens. The approach is based on tolerizing the host animal to immunodominant or otherwise undesired antigen(s) (tolerogen) that may be structurally or functionally related to the antigen of interest. Tolerization of the host animal can be achieved through one of three methods: High Zone, Neonatal, or Drug-induced tolerization. The tolerized animal is then inoculated with the desired antigen (immunogen) and antibodies generated by the subsequent immune response are screened for the desired antigenic reactivity. Over the past 15 years a large number of investigators have used the subtractive approach with cleverly chosen tolerogen-immunogen combinations and successfully generated uniquely reactive antibodies which are often neutralizing or function-blocking. This review will focus on the implementation of subtractive immunization for the production of antibodies otherwise unobtainable by standard immunization.

Subtractive immunization: a tool for the generation of discriminatory antibodies to proteins of similar sequence

Antibodies specific for a protein of interest are invaluable tools for monitoring the protein's structure, location and activity. Due to the tendency of an immune system to mount a response toward the abundant, immunodominant epitopes in a protein mixture, difficulties are inherent in the isolation of antibodies specific for proteins that are rare or poorly immunogenic. Likewise, isolation of antibodies specific for a protein with significant sequence similarity to other proteins, such as those derived from protein engineering, may be challenging. Subtractive immunization is a technique proven to facilitate efforts to produce monoclonal antibodies specific for antigens that are present in low abundance in a protein mixture, poorly immunogenic and/or similar in sequence or structure to other proteins. This protocol provides a detailed, stepwise procedure for the isolation of antibodies specific for a protein with sequence similarity to other proteins. As an example, we describe methods established to isolate antibodies specific to a methionine-enriched variant of soybean vegetative storage protein beta (VSPbeta-Met) that shares 91.8% amino acid sequence identity to the wild-type protein (VSPbeta-WT). These methods include cyclophosphamide-induced immunosuppression of mice for the wild-type protein followed by immunization with VSPbeta-Met. As a result of this procedure, mouse polyclonal antibodies that exhibited 10-fold greater reactivity with VSPbeta-Met than VSPbeta-WT in an ELISA were generated. It is anticipated that this strategy will have utility for generating antibodies specific to protein variants derived from protein engineering.

Strategies to generate antibodies capable of distinguishing between proteins with >90% amino acid identity

Protein engineering is a common strategy for the generation of protein variants with new properties. The engineered variants often have a high degree of similarity with the wild-type progenitor protein, necessitating a tool (e.g., antibody) to distinguish the wild-type and variant protein forms. As part of an overall effort to understand the process of incorporation of amino acids into storage proteins during seed fill in soybean, we have engineered a variant of soybean vegetative storage protein beta (VSPbeta) that is 91.8% identical in amino acid sequence to the wild-type protein, but contains 10% methionine (VSPbeta-Met, unpublished results). Thus, it would be desirable to have antibodies that specifically recognize VSPbeta-Met over the endogenously expressed wild-type protein in transgenic plants. To this end, we compared three strategies for the isolation of VSPbeta-Met-specific antibodies: (1) hybridoma production using VSPbeta-Met protein as the antigen, (2) polyclonal antibody production in rabbits using a peptide antigen corresponding to a methionine-rich region of VSPbeta-Met, and (3) subtractive immunization in mice using VSPbeta-WT as the tolerogen, cyclophosphamide for immunosuppression and VSPbeta-Met as the immunogen. While the first strategy generated antibodies cross-reactive to both antigens, the second strategy generated polyclonal antibodies that preferentially recognized the variant protein in immunoblots. However, using subtractive immunization, we were able to generate mouse polyclonal antibodies that exhibited 10-fold greater reactivity with VSPbeta-Met than VSPbeta-WT in an ELISA.

Peripheral tolerance mechanisms prevent the development of autoreactive T cells in chimeras grafted with two minor incompatible thymuses

The thymus has been shown to play an important role in the generation of T cell tolerance to self antigens. Developing T cells are readily tolerized to antigens which are expressed in the thymus, and it is generally thought that such thymic tolerance occurs by a mechanism of clonal deletion. We sought to examine whether T cells which initially encountered a "self antigen" post-thymically would be rendered tolerant of that antigen, and if so whether the mechanism of tolerance induction would differ from that found for thymic antigens. We constructed bone marrow radiation chimeras which were grafted with two thymus lobes differing in minor histocompatibility antigens. T cells which matured in one thymus would be tolerized to the minor histocompatibility antigens expressed in that thymus but would not encounter, and would therefore have no early opportunity of being tolerized to the minor histocompatibility antigens expressed by the other thymus. The initial encounter with the minor antigens on the second thymus would occur post-thymically. Would these T cells be tolerant or responsive to those minor histocompatibility antigens? We found that tolerance was dominant in these chimeras. The data further suggest that the mechanism responsible for tolerance induction in the periphery may differ from that which operates in the thymus.

Tolerance in rats by transplacental transfer of Dipetalonema viteae microfilariae: recognition of putative tolerogen(s) by antibodies that inhibit antigen-specific lymphocyte proliferation

We have previously reported (Nature 1982. 299:361) that the transplacental transfer of Dipetalonema viteae microfilariae (mf) can induce an antigen-specific tolerance in rats. Rats thus tolerized have serum factor(s) which block(s) antigen-specific lymphocyte proliferation. The results of experiments involving fractionation of antisera from tolerant animals indicate that the inhibitory activity for antigen-specific blastogenesis resides in IgG antibodies. Absorption of IgG (eluted from protein A) with specific filarial antigens reduced the inhibition from 58% to 9% whereas a similar immunosorption of IgG size fraction (obtained by applying to AcA 34 Ultrogel) resulted in a decrease from 72% to 35%. This suggests that IgG size fraction might include factor(s) derived from mf and was partially blocking the blastogenic response. Since the tolerant animals harbor only mf, we have used radiolabeled mf surface antigens for immunoprecipitation by antisera from tolerant animals. Antibodies from tolerant animals have a different specificity for filarial antigens compared to those from immunocompetent and mf-resistant rats.

Self-tolerance eliminates T cells specific for Mls-modified products of the major histocompatibility complex

In mice the product of the Mlsa locus is an unusual antigen capable of interaction with certain products of the major histocompatibility locus (MHC) to form a ligand for a large portion of the T-cell alpha/beta receptor repertoire, including nearly all receptors that use V beta 8.1. The presence of Mlsa/MHC during T-cell development results in the deletion of T cells that express V beta 8.1, documenting the importance of clonal deletion in establishing tolerance to self antigens.

In vivo H-2K and H-2D antigen expression in two allogeneic mouse tumours of low immunogenicity

The B16 melanoma of C57BL/6 mice immunizes very poorly, even against its own major histocompatibility complex (MHC) antigens. B16 cells expressed both H-2K and H-2D antigens in vitro as judged by binding of monoclonal antibodies to these antigens in indirect immunofluorescence staining. The in vivo MHC antigen expression of B16 was examined and compared with that of a second C57BL/6 tumour, the Lewis lung carcinoma (3LL), whose defective immunogenicity has been attributed to a selective deficiency in H-2K antigen expression. We found that 125I-labelled cells of both tumours expressed sufficient allo-antigen in vivo to be lysed in BALB/c mice which had been pre-immunized with C57BL/6 lymphoid cells. 125I-B16 cells were also lysed in MHC-recombinant mice which had been immunized against either H-2Kb or H-2Db, indicating that B16 cells express both of these MHC antigens in vivo. This contrasted with our findings with 125I-3LL cells which were destroyed in mice immunized against H-2Db but not in those immunized against H-2Kb. Thus, B16 illustrates a different deficiency in tumour cell immunogenicity which appears not to be attributable to an absence of either of the class I MHC antigens.

Regulated expression of a murine class I gene in transgenic mice

The major histocompatibility complex class I genes play an essential role in the immune presentation of aberrant cells. To gain further insight into the regulation of the expression of these class I genes and to better define the functions of their protein products, we made use of the technique of gene transfer into the germ line of inbred mice. With the use of locus-specific DNA probes, we observed that a transgenic class I gene was expressed in a tissue-dependent fashion analogous to that of an endogenous class I gene. In addition, the level of expression of the transgenic gene was substantially higher that that of the endogenous gene. The availability of transgenic mice properly expressing a foreign murine class I gene provides a unique system to further define the role of the class I antigens in the maturation of the immune response and in determining the malignant and metastatic phenotypes of tumor cells.

Differential expression of the HLA-B7 and the HLA-A2 gene in transfected mouse L(tk-) cells after stimulation by mouse interferon

Mouse L(tk-) cells were transfected with recombinant genomic clones encoding the human major histocompatibility antigens HLA-A2 or HLA-B7. The exposure of 15 different transfected cell clones to mouse interferon resulted in an up to 2.9-fold enhancement of the HLA-A2 antigen at the cell surface but in an up to 5.5-fold enhancement of the HLA-B7 antigen as shown by quantitative radioimmunoassay with monoclonal antibodies directed against different HLA epitopes. Using the HLA-Bw6 specific monoclonal antibody 2BC4, an even higher increase of the HLA-B7 antigen (up to 12-fold) could be observed. This higher inducibility of an HLA-B versus HLA-A locus gene may reflect distinct regulatory mechanism controlling the expression of HLA class I subregion antigens.

The use of tolerization in the production of monoclonal antibodies against minor antigenic determinants

An initial attempt to prepare monoclonal antibodies specific for the Dictyostelium discoideum lysosomal enzyme beta-glucosidase was unsuccessful. All of the antibodies resulting from this fusion recognized an extremely immunogenic epitope that is present on all of the lysosomal enzymes of Dictyostelium. In two succeeding fusions, changes in the immunization schedule intended to increase the immune response to enzyme-specific epitopes were not entirely successful. Although nine hybridomas producing antibodies specific for beta-glucosidase resulted from these two fusions, most (70%) of the cell lines isolated secrete antibodies that recognize the shared, immunodominant epitope. Moreover, the nine beta-glucosidase-specific antibodies proved to be of limited utility since none recognize the native enzyme. Therefore, we attempted to tolerize a BALB/c mouse to the common epitope by injecting the lysosomal enzyme, N-acetylglucosaminidase, within 40 h after birth. As an adult, this animal was immunized with beta-glucosidase. Fusion of the spleen cells from this mouse with myeloma cells resulted in the isolation of nine hybridoma lines that produce antibodies specific for beta-glucosidase. No antibodies reactive with the common epitope were detected. These results suggest that tolerization may provide a means whereby an undesired class of antibody-producing cell lines can be selectively eliminated from the products of a fusion.

Regulated expression of a murine class I gene in transgenic mice

The major histocompatibility complex class I genes play an essential role in the immune presentation of aberrant cells. To gain further insight into the regulation of the expression of these class I genes and to better define the functions of their protein products, we made use of the technique of gene transfer into the germ line of inbred mice. With the use of locus-specific DNA probes, we observed that a transgenic class I gene was expressed in a tissue-dependent fashion analogous to that of an endogenous class I gene. In addition, the level of expression of the transgenic gene was substantially higher that that of the endogenous gene. The availability of transgenic mice properly expressing a foreign murine class I gene provides a unique system to further define the role of the class I antigens in the maturation of the immune response and in determining the malignant and metastatic phenotypes of tumor cells.

Abrogation of metastatic properties of tumour cells by de novo expression of H-2K antigens following H-2 gene transfection

H-2 gene transfection was used to restore expression of H-2K antigens in metastatic and non-metastatic subclones of a murine fibrosarcoma that lack their major histocompatibility complex-encoded H-2K antigens. De novo expression of H-2K reduced tumorigenicity and abolished the formation of metastasis in syngeneic mice. Expression of H-2K may lead to effective recognition of the disseminating tumour cells by the host immune system.

The differential expression of H-2K versus H-2D antigens, distinguishing high-metastatic from low-metastatic clones, is correlated with the immunogenic properties of the tumor cells

Two clones of the 3LL Lewis lung carcinoma, a low-metastatic clone A9 and a high-metastatic clone D122, were studied for MHC expression and immunogenic properties. Using monoclonal antibodies, we demonstrated that the A9 clone expresses both the H-2Kb and the H-2Db, whereas the D122 expresses only the H-2Db, and lacks the expression of the H-2Kb encoded molecules. Cells of the low-metastatic clone A9 grew progressively in syngeneic (C57BL/6J) or in F1 mice, but were rejected in allogeneic recipients. The high-metastatic D122 grew progressively in all mouse strains tested, yet metastases were formed only in syngeneic recipients. When H-2 recombinant mice were used, the A9 again manifested a significantly greater immunogenic potency than the metastatic D122, which grew in all 4 recombinants tested. Metastases, however, were formed in B10HTG and to a lesser extent in B10A(4R), thus indicating that metastasis formation is restricted by both C57BL background and H-2Db sub region. We subsequently tested whether the higher immunogenicity of the H-2Kb-positive A9 cells is expressed also in syngeneic mice, to examine whether this could account for its low metastatic phenotype. We found that immunization by A9 cells significantly inhibited the growth of a subsequent A9 graft and even of D122, yet D122 did not retard the growth of secondary D122 or A9 cells. The increased immunogenic effect was expressed also in the generation of syngeneic cytotoxic lymphocytes by A9 but not by D122 cells. We suggest that expression of H-2K molecules on the 3LL clones, immunogenicity and the metastatic phenotype are causally related in this system.

Generation and decay of the immune response to a progressive fibrosarcoma. II. Failure to demonstrate postexcision immunity after the onset of T cell-mediated suppression of immunity

This study shows that surgical removal of the meth A fibrosarcoma from its semisyngeneic host fails to result in postexcision immunity to growth of a tumor implant unless the host already has acquired a mechanism of concomitant immunity to growth of an implant. Therefore, tumor excision does not cause immunity to be generated but preserves a mechanism of concomitant immunity that already exists and which otherwise would eventually undergo down-regulation under the influence of suppressor T cells. Removal of the tumor after it has grown large enough to cause the T cell-mediated suppression of concomitant immunity does not result in the reemergence of immunity. Instead, the host remains unable to generate concomitant immunity to a second tumor for a long period of time and retains, for at least 31 d, suppressor T cells able to passively transfer suppression to appropriate recipients. Like the suppressor T cells responsible for active suppression of concomitant immunity, the suppressor T cells responsible for "memory" suppression are of the Ly-1+2- phenotype. The results indicate that progressive tumor growth results in a state of immunological tolerance of tumor-specific, transplantation antigens that can persist in the apparent absence of tumor antigens.

A diet enriched in vitamin A acetate or in vivo administration of interleukin-2 can counteract a tolerogenic stimulus

A conventional diet enriched in retinyl acetate (vitamin A acetate; VAA) or in vivo administration of exogenous interleukin-2 (IL-2) can effectively annul the otherwise tolerogenic stimulus represented by (CBA X C57BL/10ScSn) F1 cells injected intraperitoneally into newborn CBA mice. On the basis of these data and results of others, we postulate that an antigenic stimulus associated with a relative lack of IL-2 (or generally the lack of a 'secondary stimulus') can be tolerogenic rather than immunogenic. However, the tolerogenicity of the antigenic stimulus can be substantially reduced or even converted to sensitization (R. P. Cleveland & H. N. Claman, J. Immun. 124, 474-480, 1980), when the antigenic signal is appropriately associated with a concomitant or additional stimulus possibly mediated through IL-2.

Haplotype-specific suppressor T cells mediating linked suppression of immune responses elicited by third-party H-2 alloantigens

Specific suppressor T cells (Ts) were induced in vitro by incubation of mouse spleen/lymph node cells with allogeneic heat-treated cells. These Ts inhibit mixed lymphocyte reaction (MLR) in a haplotype-specific manner. Ts also suppress cell proliferation induced by third-party H-2 alloantigens provided these are expressed on the same cell surface as at least some of the H-2 antigens used for Ts activation. Ts activated by H-2 plus non-H-2 alloantigens suppress an MLR induced by irrelevant H-2 alloantigens if these are expressed on the same cell surface as the non-H-2 alloantigens used for Ts activation. Products of the H-2 region or non-H-2 alloantigens which are not able to stimulate cell proliferation do not activate Ts. These Ts are first demonstrable after 4 days of incubation of lymphoid cells with heat-treated allogeneic cells and they inhibit MLR only if added at the very beginning of the culture. Exogenous interleukin 2 does not overcome suppression and the suppression is not due to a cytotoxic effect, since heat-treated cells do not elicit cell proliferation or cytotoxic cells. Moreover, the specific Ts differ in their Thy-1+,Ly-1+,2-phenotype from Ly-2+ allospecific cytotoxic cells. Thus specific Ts could be induced in vitro, which demonstrate linked suppression for third-party H-2 alloantigens provided these are expressed on the same cell surface as the antigens used for Ts activation.

MHC imbalance and metastatic spread in Lewis lung carcinoma clones

Imbalance in the Kb and Db region encoded molecules is observed in Lewis lung carcinoma clones. The uncloned metastatic population and the D122 high-metastatic clone show no expression of H-2Kb products, while the nonmetastatic A9 clone expresses Kb products. Twenty-nine new subclones of 3LL and A9 were analyzed for D-end and K-end membrane expression, primary growth rate and metastatic spread. We show that the imbalance in H-2Kb to H-2Db is correlated with metastatic properties of a given clone, but local tumor growth is not. A "low Kb/low Db" phenotype is nonmetastatic as is a "high Kb/high Db" phenotype; a "low Kb/high Db" is highly metastatic and a "medium Kb/high Db" is moderately metastatic. We find support for this notion of imbalance in experiments on MHC modulation by interferon and retinoic acid. Interferon increases both Kb and Db expression of A9 and D122 clones yet the net increase of Db was greater than Kb. This was associated with an increase in metastasis formation. Retinoic acid increases the expression of the Db gene product on the nonmetastatic A9, clone, without apparent changes in Kb expression. This treatment shifts the A9 to a high-metastatic phenotype. The significance of this imbalance to the tumor--host relationship is discussed.

Functional clonal deletion of cytotoxic T-lymphocyte precursors in chimeric thymus produced in vitro from embryonic Anlagen

Chimeric thymus, formed by fusing the prelymphoid third pharyngeal pouches of fetal mice with fetal liver, have been allowed to develop entirely in vitro. Syngeneic and allogeneic chimeras were prepared and both types of thymus were shown to contain substantial numbers of functional cytotoxic T lymphocyte precursors reactive against "third party" alloantigens. However, alloreactivity specific for H-2 antigens present on either the third pharyngeal pouch or the fetal liver was minimal. In three different allogeneic chimeric thymuses, the frequencies of cytotoxic T lymphocyte precursors reactive to H-2 antigens present on the third pharyngeal pouches were reduced to 1%, 4%, and 0% of control values, whereas, in the one allogeneic chimera tested for alloreactivity to H-2 antigens present on the fetal liver, the cytotoxic T lymphocyte precursor frequency was reduced to less than 1% of control values. The phenotype of the H-2 tolerance is shown to be one of functional clonal deletion of the cytotoxic T lymphocyte precursor.

Transplacental transfer of rodent microfilariae induces antigen-specific tolerance in rats

Microfilariae are the smallest form in the life-cycle of filarial nematode parasites. They are released by the adult female worms and migrate through the blood and extracellular fluids where they can be transmitted by vectors. A few reports have indicated the possibility of the transmission of microfilarial infection from mother to offspring. We have infected rats with adult females of the rodent filaria, Dipetalonema viteae, and report here that the transfer to D. viteae microfilaria does indeed occur through the placenta. Exposure to specific antigens early in development can readily induce immune tolerance. We observed that a state of reversible immune unresponsiveness occurred in rats as a result of pre- and post-natal exposure to microfilariae and this was associated with impairment of T-cell responses. The induction of tolerance allowed D. viteae infective larvae to reach maturity in the Fischer rat which is otherwise innately resistant to this practice.

Spleen cells from adult mice given total lymphoid irradiation or from newborn mice have similar regulatory effects in the mixed leukocyte reaction. I. Generation of antigen-specific suppressor cells in the mixed leukocyte reaction after the addition of spleen cells from adult mice given total lymphoid irradiation

We added spleen cells from adult BALB/c mice treated with total lymphoid irradiation (TLI) to the mixed leukocyte reaction (MLR) using a variety of responder and stimulator cells. The spleen cells nonspecifically suppressed the uptake of [3H]-thymidine and the generation of cytolytic cells regardless of the responder-stimulator combination used. We also examined the effect of the spleen cells on the generation of antigen-nonspecific and antigen-specific suppressor cells in the MLR. The experimental results suggest that the spleen cells from TLI-treated mice inhibit the generation of nonspecific suppressor cells, but do not inhibit the generation of antigen-specific suppressor cells. Thus, alloantigenic stimulation of normal responder cells in vitro in the presence of spleen cells from TLI-treated mice generates large numbers of antigen-specific suppressor cells, but few cytolytic cells or nonspecific suppressor cells. Similar nonspecific inhibition of the MLR was observed with neonatal spleen cells. This in vitro system provides a regulatory model for the induction and maintenance of tolerance in vivo, in which adult mice given TLI or neonatal mice accept allogeneic bone marrow transplants without graft-vs.-host disease.

Immunogenetic aspects of in vivo allotolerance induction during the ontogeny of Xenopus laevis

This study describes the ontogeny of allograft immunity in a partially inbred strain of frogs (Xenopus laevis). At various times during the frogs' premetamorphic, perimetamorphic, and postmetamorphic life, major histocompatibility complex (MHC) homozygous strain JJ Xenopus (MHC haplotype j) were grafted with skin from adult donors of defined MHC homozygous (j,f) and heterozygous (j/f,f/h) haplotypes. This protocol reveals that destructive allograft reactivity to MHC alloantigens in Xenopus matures slowly and that allotolerance can be induced to such MHC-encoded antigens throughout larval life. The ultimate fate of an MHC disparate transplant (survival or rejection) is dependent on several interacting variables, which include antigen dose, haplotype dose, and the developmental stage of the host frog at the time of transplantation. In contrast, minor H-locus disparate (MHC compatible) grafts never appear to be rejected by hosts grafted at any larval stage.

The toleration of rat thymocytes to xenogeneic erythrocytes: kinetics of induction and recovery

The intravenous injection of large doses of xenogeneic erythrocytes into rats completely abrogated the ability of thymocytes from such animals to provide help for antierythrocyte antibody responses in an adoptive transfer system. Thymocyte tolerance developed at a time when the thymocyte donors were producing antibody to the tolerizing antigen, and spleen helper T cells were not tolerized by the injection protocol. A minimum of two injections of 1 ml of packed erythrocytes, spaced 4 - 5 days apart, was required to induce loss of helper activity. Tolerance was fully developed within 16 h of the last injection of antigen. Helper activity recovered over a 24-h period starting 5 days after this injection but could be delayed by giving further injections of antigen. These results suggest that the intrathymic pool of cells with assayable helper activity turns over in about one day and that precursors of this pool are probably also tolerable. Suppressor cells could not be detected in tolerant thymocyte populations indicating that the loss of helper activity was either a consequence of clonal inactivation or that an antigen-specific mechanism existed for the recruitment of helper cells from the thymus. The significance of these findings to the acquisition of self-tolerance is discussed.

Selective induction of immunological tolerance in antiviral T killer cells of inbred mice after treatment with cyclosporin A

Primary anti-influenza A cytotoxic thymus-derived (T) and bone marrow (B) lymphocyte-dependent responses in inbred mice were used as an in vivo model system to study the effects of the immunosuppressive fungus metabolite cyclosporin A (CyA). Five consecutive daily oral applications of CyA, with the first being given 1 or 2 h before virus inoculation of the animals, caused a complete blockage of induction of anti-influenza T killer cells and a partial reduction of cytotoxic B lymphocyte activities. Adoptive cell transfer experiments revealed that incapability to respond was due neither to humoral factors nor to the generation of suppressor cells. The tolerance state appeared to be specific for influenza A; cytotoxic T lymphocytes against allogeneic cell surface determinants could be stimulated in immunosuppressed mice. CyA treatment abolished virus-specific and cross-reactive anti-influenza killer T cell responses. Suppression was of short duration: less than 1 week for B cell-dependent functions, and between 1 and 2 weeks for T killer cell responses. Animals appeared to be normal with regard to both of these cellular activities for 4 weeks after tolerance induction. Thus, the data indicate that CyA exerted preferential effects on killer T cells. Moreover, evidence was presented that CyA treatment during an ongoing influenza infection did not increase sensitivity to that virus. Mice with no measurable cytolytic anti-influenza T killer cell activities but significant B cell responses, although partially diminished by the drug, were completely protected against the lethal effects of influenza infection.

Serological detection of H-2K- and H-2D-gene products. I. Principal difference between T and B lymphocytes in expression of H-2D-encoded alloantigens

Using the fluorescence-activated cell sorter (FACS II), we have analyzed the expression of H-2K- and H-2D-gene products on the membrane of various cellular components of the murine immune system. Using this serological technique we show a basic difference between T and B lymphocytes. Whereas all cellular components analyzed--hydrocortisone-resistant thymocytes, splenic T and B lymphocytes, macrophages and bone-marrow cells--expressed H-2K-subregion-encoded alloantigens at a high density, it seems that the high density expression of H-2D-encoded alloantigens is restricted mainly to B cells and to macrophages. Hydrocortisone-resistant thymocytes, splenic T lymphocytes and bone-marrow cells, on the other hand, showed significant expression of the H-2D alloantigens only at low membrane density. These results, then, provide evidence for the existence of an imbalance in serologically detectable expression of H-2K- and H-2D-region-gene products on the cell membrane of various cells comprising the murine immune system.