Monoclonal antibodies directed against T cell epitopes presented by class I MHC antigens.

Monoclonal antibodies which recognize a synthetic peptide derived from influenza virus nucleoprotein in association with a murine class I MHC molecule (Kd) have been isolated. One such antibody has been characterized and shown to react neither with the peptide nor with the Kd molecule, but only with the Kd-peptide complex. Evidence is given that it recognizes the naturally processed peptide located in the peptide binding groove, i.e. the antigenic moiety presented to T cells.

Induction and suppression of delayed-type hypersensitivity to non-MHC antigens during lethal graft-versus-host reaction.

A delayed-type hypersensitivity reaction (DTH) to non-MHC Ag was detected during a lethal graft-vs-host reaction (GVH) induced by incompatibility for non-MHC Ag alone. In this model, when appropriate doses of B10.D2 bone marrow and lymphoid cells are grafted to irradiated (DBA/2 x B10.D2)F1 recipients, the ensuing GVH, directed against those DBA/2 non-MHC Ag absent from the B10.D2 background, results in virtually 100% mortality in less than 3 mo; donor alloimmunization against the host histocompatibility Ag considerably reduces mortality, and survival rates of 80 to 100% are common. The experiments reported here show that: 1) the cell responsible for DTH induction expresses the CD4+ CD8- phenotype; 2) CD4+ cells likewise seem to play a predominant role in the pathology of lethal GVH in this genetic combination; 3) the alloimmunization protocol that abrogates mortality also abolishes GVH-associated DTH; and 4) this suppressive effect, as shown elsewhere for the protection against mortality, is mediated by CD4- CD8- "double negative" Thy-1+ CD3+ T suppressor cells. Thus, there is a good parallel between lethal GVH and its associated DTH as concerns both induction and suppression of the two phenomena, suggesting that mortality and DTH may represent different manifestations of a common underlying mechanism. Initiation of the effector phase of DTH in the adoptive transfer model seems to be dependent on the presence of a Thy-1+ double-negative cell in the transfer inoculum; the possible relationship of this double-negative cell to the Th-1-type CD4+ DTH-mediating cell recently shown to induce lethal GVH is discussed.

Mls-1a-induced peripheral tolerance to host minor histocompatibility antigens in radiation bone marrow chimeras. Modification of T cell repertoire associated with active suppression and permanent presentation of host antigens.

Minor histocompatibility Ag (mHAg) can be responsible for the development of graft vs host reaction (GVHR) after bone marrow transplantation. In a mouse model, B10.D2 donor immunization against Mls-1a prevents lethal GVHR developed by CD4+ T cells against DBA mHAg in irradiated (DBA/2 x B10.D2)F1 hosts. Such F1 hosts become 100% chimeric and show long term survival (LS mice). The cellular mechanisms underlying the tolerance in LS mice was investigated. It was found that a state of tolerance can be induced in thymectomized F1 hosts. Although spleen cells from LS mice are able to initiate lethal GVHR in third-party H-2k-incompatible hosts, no GVHR is observed in secondary hosts incompatible for specific DBA/2 mHAg. Mixed lymphocyte experiments in vitro confirm that T cells from LS mice are unresponsive toward specific DBA/2 mHAg, although they are able to proliferate in response to H-2 or Mls-1a Ag. The responsiveness to Mls-1a correlates with the presence of V beta 6+ cells in LS mice, probably derived from mature T cells present in the donor inoculum. The tolerance in LS mice is not due to the lack of DBA/2 mHAg presentation; instead, permanent presentation of Ag (Ag I and Ag II) previously described as being responsible for lethal GVHR is consistently observed. A significant protection against GVHR is obtained by transferring normal B10.D2 cells together with spleen cells from LS mice, clearly indicating the contribution of active suppression in the state of tolerance; this is further confirmed by in vitro results obtained in limiting dilution assays. It is concluded that tolerance in chimeric LS mice 1) is due to a peripheral (thymus-independent) mechanism; 2) is specific for mHAg; 3) correlates with unresponsiveness of the repertoire to host mHAg, without alteration of the repertoire for H-2 and Mls-1a Ag; and 4) is associated with an active suppression and with a permanent presentation of at least two mHAg responsible for GVHR mortality.

Mechanism of hybrid resistance. The role of a natural antibody in parental bone marrow cell rejection.

Hybrid resistance (HR) to parental bone marrow growth is specifically directed against hemopoietic histocompatibility (Hh-1) Ag that are present in parental bone marrow cells (bmc). The mechanism of HR seems to be a multistep process. According to a model we proposed earlier, a T cell recognizes the Hh-1 Ag and stimulates a macrophage to secrete IFN-alpha/beta (recognition phase). IFN-alpha/beta activates a NK-like cell that specifically kills the parental bmc (effector phase). We have also described in a previous paper that serum from resistant F1 hybrids contains a humoral factor that seems to be involved in the effector phase of HR. In the present work, we study the role and the nature of this humoral factor. Our results show that this humoral factor: 1) is present in all resistant H-2Db heterozygous F1 hybrids we have tested but not in nonresistant H-2Db homozygous mice; 2) seems to recognize the Hh-1b Ag because it is absorbed on bmc from Hh-1b mice but not on bmc from Hh-1d and Hh-1- mice; and 3) is an IgG1 Ig (natural antibody). These results could help us to explain the specificity of HR at the effector phase by supposing that this natural antibody recognize the Hh-1 Ag and enable NK-like cells to kill parental bmc cells in Hh-1 specific manner.

Mlsa generated suppressor cells. I. Suppression is mediated by double-negative (CD3+CD5+CD4-CD8-) alpha/beta T cell receptor-bearing cells.

Grafting of cells from B10.D2 (H-2d) donors into H-2 compatible lethally irradiated (DBA/2 x B10.D2)F1 hosts results in a severe graft-vs-host reaction (GVHR), developed against DBA/2 non-H-2 Ag, with only 0 to 10% of animals surviving. This GVHR mortality rate is dramatically reduced (90 to 100% of animals survive) by donor preimmunization against Mlsa determinants. The protection against GVHR correlates with a decreased B10.D2 anti-DBA/2 proliferative response in vitro. Both in vivo and in vitro phenomena are associated with activation of CD5+ suppressor T cells in the spleens of immunized mice. The present work was designed to study the origin of these suppressor cells and to further characterize their phenotype. The results show that significant suppression is not inducible in "B" mice. In contrast, in mice that were only thymectomized or else pretreated in vivo with anti-CD4 or anti-CD8 mAb, the suppressor cells are activated as efficiently as in normal mice. The suppression of GVHR mortality and proliferative responses in vitro is lost after depletion from preimmunized splenocytes of CD5+ T cells and remains unaltered after depletion of CD4+ or CD8+ T cells or both. Depletion of asialo GM1+ cells removes all NK activity, whereas the suppression is decreased only slightly. FACS analysis showed that double-negative (DN) cells from normal and immunized mice contain both CD3+ and CD3- cells; the vast majority of the CD3+ DN T cells express the alpha/beta T cell receptor. Suppression of GVHR and of proliferative responses in vitro are abrogated after elimination of CD3+ cells. These results suggest that Mlsa generated suppressor cells: 1) are derived from post-thymic long-lived T cell precursors; 2) are low asialo GM-1+ but do not exhibit NK activity; 3) belong to a subset of peripheral CD5+ DN T cells bearing a CD3-associated alpha/beta-heterodimer.

Graft-versus-host mortality induced by noncytolytic CD4+ T cell clones specific for non-H-2 antigens.

UNLABELLED: The relative contribution of individual non-H-2 Ag and of T cell subsets that initiate graft-vs-host reaction (GVHR) as well as the mechanism responsible for histopathologic lesions are still a matter of debate. To address these questions and to favor the selection of T cells primed in vivo against non-H-2 Ag important in GVHR we derived T cell clones from spleens of (DBA/2 x B10.D2)F1 (H-2d) mice developing this reaction after the graft of B10.D2 (H-2d) cells incompatible for numerous non-H-2 Ag plus Mlsa. The pattern of reactivity of eight selected clones against cells from different strains of mice including (BXD)RI strains indicated that one CD4+ clone is specific for Mlsa and seven additional clones (six CD4+ and one CD8+) are specific for four different non-H-2 Ag (Ag.I-IV) and proliferate in an H-2-restricted manner. The same series of experiments suggested that Ag.I and II are poorly polymorphic and allowed to propose the localisation of the genes controlling Ag.I (chromosome 1) and Ag.III (chromosome 4). All the clones show a triple (alpha, beta, gamma) mRNA transcript for TCR but at their surface they express the alpha/beta-heterodimer. The clone specific for Mlsa expresses V beta 6 and that specific for Ag.IV expresses V beta 8.1. Rapid mortality accompanied by clinical and histologic signs of severe GVHR was observed after administration of CD4+ clones (together with host-syngeneic bone marrow) derived early after grafting and specific for Ag.I and II but not after administration of: 1) CD8+ cytolytic clone derived early after grafting and specific for Ag.IV; 2) CD4+ clones derived late after grafting and specific for Ag.III; and 3) CD4+ clone specific for Mlsa. A clear correlation was established between the capacity of CD4+ clones to induce GVHR mortality, to mediate host-specific DTH and to release a high level of TNF. IN CONCLUSION: 1) the reaction against a single non-H-2 Ag is sufficient to provoke lethal GVHR; 2) the capacity to provoke GVHR mortality depends on antigenic specificity and functional properties of the responding clones; 3) the inflammatory process mediated by CD4+ clones may play a major role whereas the specific CD8+ T cell-mediated cytolytic activity is not necessarily lethal.

Specific inhibition of hybrid resistance in F1 hybrid mice pretreated with parent-strain spleen cells. II. Evidence for an Hh-1 antigen-specific tolerization.

Lethally irradiated F1 mice reject bone marrow graft from H-2b parents. In a previous paper we showed that pretreatment of F1 hybrid with H-2b parental spleen cells abrogates this hybrid resistance (HR) to parental bone marrow growth by inducing a Thy-1+Lyt-1+2- nylon-adherent suppressor cell. We studied the mechanism of induction of this suppressor cell. Two hypotheses were tested; both were based on the observation that parental spleen cells when injected into a F1 hybrid, recognize the alloantigens of the opposite parent and proliferate; the proliferation of these Hh-1+ cells may result in an overload of the pretreated F1 hybrids with Hh-1 Ag, and in the development of a graft-vs-host reaction that is followed by a non-specific immunodeficiency (GVHID). Thus abrogation of HR could be due to either a tolerization with high doses of Hh-1 Ag or the GVHID. Our results show that abrogation of HR does not correlate with the GVHID because 1) it is induced after pre-treatment with H-2b parental cells only, whereas GVHID is observed after injection with cells from either of the two parents; and 2) it is induced in several conditions where GVHID does not occur; after pre-treatment with 1000-rad-irradiated or T-cell depleted or only class I incompatible spleen cells or with spleen cells from nude parents as well as after pre-treatment with H-2b bone marrow cells. HR is overcome by the injection of H-2Db homozygous or of cross-reactive H-2Ds homozygous cells only. However, although pretreatment with H-2Db homozygous spleen cells is necessary, it is not sufficient for an efficient overcoming of HR. Indeed enhancement of H-2b bone marrow growth after pre-treatment with 1000-rad-irradiated, T-cell depleted or nude parent spleen cells is very short-lasting and never reaches the level observed after pre-treatment with normal spleen cells. We conclude that inhibition of HR in F1 hybrids pretreated with parental spleen cells is not a consequence of a GVHID but of a specific tolerization with Hh-1 Ag; however, the HR is inhibited more consistently when inoculum used for the pretreatment contains fully immunocompetent T cells. The role of the immunocompetent parental T cells in abrogation of HR is discussed.

Abrogation of lethal graft-versus-host reaction directed against non-H-2 antigens: role of Mlsa and K/I region antigens in the induction of unresponsiveness by alloimmunization.

A graft-versus-host reaction (GVHR) directed against DBA/2 non-H-2 antigens alone can be induced by grafting B10.D2 bone marrow and spleen cells intravenously to heavily irradiated, H-2d compatible (DBA/2 X B10.D2)F1 adult mice. Under the experimental conditions used, only 0-10% of recipients survive, but the survival is greatly increased by donor alloimmunization, a few days prior to grafting, against host-specific (DBA/2) non-H-2 antigens and non-specific (foreign) H-2 antigens. The increased survival is mediated by alloimmunization-activated suppressor cells which can decrease the intensity of the immune reaction developed by normal B10.D2 cells both in vivo (GVHR) and in vitro (proliferative response measured in mixed lymphocyte culture, MLC). The present experiments were designed to explore the antigenic requirements for inducing suppression. The results showed that in GVHR the protective effect induced by donor alloimmunization against the specific non-H-2 antigens, which leads to 70-80% survival, is due primarily, if not entirely, to immunization against Mlsa antigens. Results of MLC experiments confirmed this conclusion, showing that immunization against Mlsa antigens is sufficient to account for the suppressive effect induced by the specific immunization. In addition, they indicated that the non-specific protective effect induced by donor alloimmunization against foreign H-2 antigens, which leads to 20-30% survival, is due to immunization against antigens encoded by the K and/or I region(s) of the H-2 complex; immunization against D region encoded antigens alone has no effect.

Parameters involved in the induction and abrogation of the lethal graft-versus-host reaction directed against non-H-2 antigens.

The grafting of cells from donors incompatible for non-H-2 antigens alone can lead to GvHR mortality in up to 100% of lethally irradiated adult recipients. GvHR severity correlates with the number of mature immunocompetent cells present in the bone marrow inoculum. Histologic and clinical manifestations of GvHR observed in these mice differ from those seen when GvHR is induced across an H-2 barrier. The number of non-H-2 genes capable of influencing GvHR mortality is probably great, and their effects may vary as a function of sex. The non-H-2 genes influence GvHR mortality mainly via their interactions, the consequences of which are complex and can result in either cumulative or suppressive effects. GvHR mortality is considerably reduced by donor immunization, shortly before grafting, against host-specific non-H-2 antigens; and it is virtually abrogated by an additional immunization of the donors against nonspecific (foreign) H-2 antigens. Three weeks after grafting, these "protected" mice are easily distinguishable from those undergoing lethal GvHR, as assessed by both clinical appearance and histologic examination; in contrast, they are nearly indistinguishable from control mice grafted with syngeneic cells. However, depending upon the conditions used for the immunization, an additional immunization against nonspecific H-2 antigens can lead to acceleration rather than suppression of GvHR mortality; this phenomenon is not seen, under the same experimental conditions, after immunization against specific non-H-2 antigens alone. It is therefore suggested that a "second signal" provided by an additional nonspecific stimulus can potentiate either the establishment of specific suppression or the activation of a secondary ("positive") response. Suppressive effects of the specific and nonspecific immunizations are cumulative, and both treatments activate suppressor cells. The intensity of suppression induced by both specific and nonspecific immunizations is antigen dose-dependent. At equivalent antigen doses the specific immunization is considerably more effective than the nonspecific immunization, and is detectable after injection of as few as 2.5 X 10(5) cells. In both cases, irradiation of the immunizing cells abolishes the suppression induced by the lower cell doses tested, while it merely decreases the intensity of the suppression induced by the higher cell doses tested. The impairment of suppression after irradiation of the immunizing cells is not attributable to a modification of their homing pattern, but to the fact that proliferation of the immunizing cells, which leads to an augmentation of the antigen dose, is abolished by irradiation.(ABSTRACT TRUNCATED AT 400 WORDS)

Soluble Mlsa antigens: stimulatory effect in vitro versus suppressive effect in vivo.

Using a pair of congenic strains of mice differing only at the Mls haplotype (Mls locus and closely linked genes), BALB/c (Mlsb) and BALB.D2-Mlsa, we have compared the in vitro proliferative responses of Mlsb lymphocytes to Mlsa antigens presented on either lymph node cells (LNC) or peritoneal adherent cells (PAC). Results showed that Mlsa-PAC are stronger stimulators than Mlsa-LNC, and furthermore, that the supernatant from Mlsa-PAC may be effective in eliciting a lymphocyte proliferative response. The proliferation in response to PAC supernatant is partially due to activation by nonspecific factor(s); however, the response in the presence of Mlsa incompatible PAC supernatant is about three times greater than the response obtained in the presence of syngeneic Mlsb-PAC supernatant, suggesting an additional stimulation by soluble Mlsa antigens. Contrasting with the ability of PAC-supernatant to stimulate a primary proliferative response in vitro, the in vivo immunization of Mlsb mice with Mlsa-PAC supernatant abrogates the specific proliferative response in subsequent one-way mixed lymphocyte cultures. This abrogation of the specific response is comparable to that observed after immunization with intact Mlsa peritoneal or spleen cells, although in the latter case the anti-H-2 proliferative response is also decreased, regardless of whether the H-2 incompatible stimulating cells express an additional incompatibility for Mlsa. The proliferation of untreated, but not of Mlsa-immunized BALB/c LNC, is stronger in cultures with DBA/2 stimulating cells (incompatible for Mlsa and other non-H-2 antigens) than in cultures with BALB.D2-Mlsa cells (incompatible for Mlsa alone), and is comparable in intensity to that activated by H-2 incompatibility. We conclude that Mlsa antigens are more efficiently recognized by unprimed helper T cells when presented on PAC than when presented on LNC. In the primary proliferative response, the effects of Mlsa and other non-H-2 antigens may be cumulative. In vivo immunization against Mlsa antigens results in suppression of the specific proliferative response and, to a certain extent, of the nonspecific proliferative response (directed against both H-2 and other non-H-2 antigens). Since Mlsa antigens are obtainable in soluble form, their physico-chemical purification can now be envisaged.

Alloimmunization-activated suppressor cells. II. In vitro activity of suppressor cells implicated in the abrogation of lethal graft-versus-host reaction.

Incompatibility for DBA/2 (D2) minor histocompatibility antigens (MiHA) alone leads to a severe lethal graft-versus-host reaction (GVHR) in irradiated (D2 X B10.D2)F1 mice receiving a bone-marrow-plus-spleen-cell graft from B10.D2 donors. This mortality is abrogated when the donors are preimmunized simultaneously against specific D2 MiHA and unrelated H-2 antigens three days before grafting. Results presented here demonstrate that spleen cells from such preimmunized donors, which are able to diminish the intensity of the GVHR developed by normal cells, exhibit the following properties: (1) reduced proliferative response to D2 MiHA in one-way MLC; (2) radioresistant suppressive effect on the proliferative responses of both normal and specifically primed B10.D2 cells; (3) inability to lyse D2 target cells even after in vitro boosting, implying that the suppressive effect detected in vitro is not due to removal of the stimulating antigens from the culture; and (4) inability to suppress the development of an anti-MiHA cytolytic response by specifically primed B10.D2 cells, despite a pronounced suppressive effect on their proliferation. Taken together with the GVHR observations, these results suggest that simultaneous immunization against specific MiHA and unrelated H-2 antigens activates, in the spleens of treated donors, suppressor cells that can inhibit the immune response at the recognition phase, and probably also at an afferent, but not efferent, stage of cytotoxic effector-cell differentiation.

Alloimmunization-activated suppressor cells. I: Abrogation of lethal graft-versus-host reaction directed against non-H-2 antigens.

Incompatibility for DBA/2 (D2) minor histocompatibility antigens (MiHA) alone leads to a severe lethal graft-versus-host reaction (GVHR) in adult (D2 X B10.D2)F1 recipients of B10.D2 bone marrow and spleen cells. In this genetic combination, mortality is completely abrogated by preimmunization of the graft donor against unrelated H-2b antigens and specific D2 MiHA a short time before grafting. Protection against GVHR mortality is elicited by immunizing the donors with a single transfusion of incompatible spleen cells or whole blood. Abrogation of GVHR mortality is due mainly to the immunization with specific MiHA, and only to a much lesser degree to the immunization with unrelated H-2 antigens; the protective effect induced by association of these two types of immunization, however, is significantly better than that elicited by either type of immunization alone. It is unlikely that this abrogation of GVHR mortality results from "dilution" of specifically reactive cells; rather, suppressor cells appear to be a contributing factor, because the preimmunization activates, in the donor spleen, suppressor cells capable of decreasing the severity of the GVHR developed against MiHA by normal cells. Histopathologic observations indicated that the lesions induced in various tissues after grafting of preimmunized donor cells were considerably less severe than those induced by grafting of normal donor cells. However, simultaneous immunization with specific MiHA and unrelated H-2 antigens may also exacerbate the GVHR, depending upon the conditions used for preimmunization; abrogation of GVHR mortality is favored by a single immunization with unrelated H-2b antigens and specific MiHA administered simultaneously a short time before grafting, whereas an acceleration of GVHR mortality is favored by long intervals and multiple immunizations. It is suggested that, depending upon the conditions used for the preimmunization, the allogeneic effect produced by stimulation with unrelated H-2 antigens may augment the response to MiHA or it may enhance the activation of suppressor cells that contribute to the abrogation of GVHR mortality.

Strong histocompatibility and cell-mediated cytotoxic effects of a single Mls difference demonstrated using a new congenic mouse strain.

In vivo and in vitro effects of incompatibility at the Mls locus have been studied utilizing a recently created congenic mouse strain. Results obtained with skin grafts were compared to those obtained in the mixed lymphocyte reaction (MLR) and cell-mediated cytotoxicity assays. The in vitro responsiveness of cells from skin-grafted mice was compared to that of cells from corresponding ungrafted mice. The results showed that: (a) Mlsa, strongly stimulatory in primary MLR, has a weak effect on skin graft rejection; specific in vivo preimmunization against Mlsa increases and accelerates the rejection of skin grafts, but abrogates the responsiveness in MLR; and (b) incompatibility for Mlsb, nonstimulatory in primary MLR, induces relatively rapid rejection of 100% of skin grafts; this rejection is dramatically accelerated by specific in vivo preimmunization and is followed by activation of helper and cytotoxic cells. Results obtained in the cell-mediated cytotoxicity assay suggest that the recognition of Mlsb determinants is H-2-restricted. Finally, the rejection of skin grafts incompatible for numerous non-H-2 loci is delayed by an additional incompatibility for Mlsb, suggesting that Mlsb decreases the response to other non-H-2 antigens, thus acting as a suppressor and/or competitor antigen. We conclude that, in contrast with previous findings, Mls incompatibility may have a strong effect on skin graft rejection, depending on the allelic combination involved, and, after in vivo immunization, Mlsb activates cell-mediated proliferative and cytotoxic responses and definitely is not "silent". The importance of the histocompatibility effects of Mls determinants and the variety of its biological functions are much in favor of the existence of a polymorphic and complex system capable of activating different cell subsets.

Incompatibility for or pre-immunization against M1s determinants decreases lethal graft-versus-host reaction developed across non-H-2 and/or H-2 barriers.

The effect of incompatibility for M1s determinants was studied in lethal graft-versus-host reaction (GVHR) in the mouse. GVHR was induced in adult recipients of the following H-2k strains: (AKR x B10.BR)F1 (Mlsa/Mlsb); (C3H x B10.BR)F1 (Mlsc/Mlsb); (CBA/J x B10.BR)F1 (Mlsd/Mlsb) and (CBA/H x B10. BR)F1 (Mlsb). Recipient mice were heavily irradiated and grafted with bone marrow and spleen cells from H-2 compatible B10.BR (H-2k, Mlsb) or H-2 incompatible B10.D2 (H-2d, Mlsb) or B10 (H-2b, Mlsb) strains. The cells from B10.D2 and B10 donors were normal, while those from B10.BR donors were either normal or pre-immunized against the recipient strains. In all experiments the survival of recipients with Mlsa/Mlsb and Mlsd/Mlsb phenotypes, and only in one experiment of those with Mlsc/Mlsb phenotype was greater and/or the survival time longer than that of recipients expressing only Mlsb. However, late deaths (greater than 120 days post grafting) observed after grafting of normal B10.BR cells were more frequent in Mlsd/Mlsb than in Mlsb strains. On the other hand, when B10.BR donor cells were pre-immunized against H-2k compatible (AKR x B10.BR)F1 (Mlsa/Mlsb) or (CBA/J x B10.BR)F1 (Mlsd/Mlsb) strains, the survival time of H-2 incompatible (B10 x B10.BR)F1 (H-2b/k, Mlsb) recipients was longer than when donor cells were pre-immunized against (CBA/H x B10-BR)F1 (Mlsb) strain. We conclude that donor incompatibility for Mlsa or Mlsd or donor-pre-immunization against Mlsa or Mlsd exerts a protective effect on lethal GVHR developed across non-H-2 or H-2 barriers; the protective effect of Mlsc is less efficient or absent. The Mls-induced protective effect shows the following properties: (a) efficiency in vivo correlates with the capacity of the corresponding alleles to stimulate an in vitro MLR; (b) is efficient in either primary or secondary response to other minor antigens; (c) is not H-2 restricted; (d) is nonspecific; (e) disappears late after grafting; (f) with respect to the genetic background, the early protective effect is followed, late after grafting, by an opposite effect which increases the mortality, suggesting that Mls locus determinants are capable of activating several cell populations with different biological functions.

Interactions of major and minor histocompatibility antigens in the graft-versus-host reaction.

The greater severity of the graft-versus-host (GVH) reaction induced by grafting C57BL/6(B6) lymphoid or bone marrow cells to irradiated (B6 x DBA/2)F1 recipients, as compared to that induced in H-2-identical (B6 x BALB/c)F1 recipients, has been shown to arise from a synergistic effect of H-2d and DBA/2 minor histocompatibility antigens (MiHAs), and not from a difference in the intensity of genetic resistance to B6 cell grafts exhibited by the two F1s. Furthermore, detection of such synergistic effects depends upon choice of an appropriate assay system; GVH splenomegaly results cannot be used to predict the outcome of a systemic GVH reaction.