Features of T cells causing H-2-restricted lethal graft-vs.-host disease across minor histocompatibility barriers

Selective depletion of marrow-T cytotoxic lymphocytes (CD8) in the prevention of graft-versus-host disease after allogeneic bone-marrow transplantation

In vitro depletion of mature pan-T lymphocytes has been widely and successfully used to prevent acute graft-versus-host disease (GVHD) after allogeneic bone-marrow transplantation (BMT). However, this procedure has been associated with a high incidence of graft failure and leukemic relapse. In this pilot study, we evaluated the efficiency of a selective depletion of human marrow T cytotoxic lymphocytes (CD8), a subset essential to induce GVHD in mice. Eleven patients with hematologic malignancies were included (7 HLA-matched BMT, 4 HLA-mismatched BMT). Marrow treatment with 7 anti-CD8 mAbs and rabbit complement resulted in a marked reduction of CD8+ lymphocytes from 15% (median value; range 7%-31%) to 1% (median value; range less than 1%-11%). Acute GVHD was not abolished by this procedure despite postgraft immunosuppression. One patient (HLA-mismatched BMT) rejected his graft and had a full autologous recovery. In conclusion, when compared to the data in the literature, CD8 depletion was shown to be less efficient than pan-T-cell depletion in the prevention of GVHD after allogeneic BMT and was still associated with a major complication associated with this procedure, i.e., graft failure.

Reactivity of Ly-2+ T cells against 2,4,6-trinitrophenyl (TNP)-modified syngeneic stimulator cells: specificity, frequency of interleukin 2-producing Ly-2+ helper T cells and clonal segregation from Ly-2+ cytotoxic T lymphocytes

The in vitro reactivity of purified murine Ly-2+ and L3T4+ T cells towards 2,4,6-trinitrophenyl (TNP)-modified syngeneic stimulator cells was analyzed. Both T cell subpopulations autonomously proliferated and produced interleukin 2. In either the Ly-2+ or L3T4+ T cell subset the frequencies of TNP-specific interleukin 2 (IL 2)-producing T lymphocyte precursors (IL 2 TL-p) were equally high (f = 1/400-1/1000). Clonally developing IL 2 TL of either T cell subset showed an exquisite antigen (TNP) specificity as shown by the split culture approach. TNP-specific Ly-2+ IL-2 TL used class I MHC (H-2Kk) gene products as major histocompatibility complex (MHC) restriction elements, while L3T4+ IL 2 TL proved to be class II MHC (H-2I-AkI-Ek) restricted. Clonal segregation analyses revealed that the majority of clonally developing TNP-reactive Ly-2+ TL segregated into either IL 2 TL-p or cytotoxic T lymphocyte presursors, i.e. both functions appear to be mutually exclusive. Less than 10% of the responding Ly-2+ T cells seemed to be bifunctional. These findings provide compelling evidence for the L3T4+ T cell-independent, autonomous reactivity of Ly-2+ T cells in MHC-restricted antigen-specific responses and suggest T-T cell interactions within the functional heterogenous Ly-2+ T cell population.

L3T4+ T-cell-independent reactivity of Lyt2+ T cells in vivo

The aim of this study was to analyze in vivo the L3T4+ T-cell-subset-independent reactivity of Lyt2+ T cells toward transplantation alloantigens. To this end, we depleted normal mice of L3T4+ T cells by injection of monoclonal antibodies to the L3T4 antigen. This procedure not only led phenotypically to a disappearance of L3T4+ T cells, but also effectively abolished reactivity toward class II MHC antigens in vitro and in vivo. However, L3T4+ T-cell-depleted mice still reacted to class I MHC alloantigens in vivo: after immunization with class I MHC alloantigens Il-2 receptor-bearing T cells appeared in the draining lymph nodes, and developed antigen-specific cytolytic activity. Moreover, upon in vivo priming the frequencies of class I MHC-specific precursors of Il-2-producing and cytolytic Lyt2+ T lymphocytes increased up to 20-fold. L3T4+ T-cell-depleted mice rejected class I MHC-bearing skin grafts promptly. We conclude that not only in vitro but also in vivo Lyt2+ T cells remain reactive toward class I MHC antigens in the absence of L3T4+ T helper cells.

Abrogation of the lethal graft-vs.-host reaction developed to non-H-2 antigens: involvement of T suppressor cells distinct from veto cells

The mortality induced by graft-vs.-host reaction (GVHR) in (DBA/2 x B10.D2)F1 recipients transplanted with cells from H-2d-identical B10.D2 donors can be abrogated by preimmunizing the donors with parent-strain spleen cells from normal DBA/2 mice. The experiments described here were designed to explore the possibility that the observed protection might be mediated by veto cells contained in the immunizing cell inoculum; the reasoning was based on an analogy with the cytotoxic T lymphocyte response to non-H-2 antigens where suppression can be mediated by veto cells, present in the spleens of normal mice, which are radiosensitive and largely Lyt-2+. We show that the intensity of the protection against GVHR mortality is a function of the immunizing cell dose, and that protection remains effective when optimal doses of immunizing cells are (a) irradiated or (b) pretreated with anti-Thy-1 serum. GVHR suppression is abrogated when, before transfer to F1 recipients, suppressor cells from spleens of immunized donors are pretreated with antiserum directed against Lyt-1.2 (expressed by B10.D2 but not by DBA/2, which expresses Lyt-1.1); in contrast, it is not significantly affected when these same cells are pretreated with anti-Lyt-2.2 alloantiserum. We conclude that when the antigen load is great enough the immunizing cells play a largely passive role in the observed suppression. The protection against GVHR mortality seen in this H-2-compatible combination is transferable by Lyt-1+2- suppressor T cells originating in mice given high doses of alloantigen. These suppressor cells are therefore distinct from the splenic veto T cells effective against cytotoxic T lymphocyte responses to non-H-2 antigens. The mechanism of the observed suppression and its relationship to Mls product(s) are discussed.

Variable capacity of L3T4+ T cells to cause lethal graft-versus-host disease across minor histocompatibility barriers in mice

Highly purified populations of L3T4+ and Lyt-2+ T cell subsets were compared for their capacity to cause lethal GVHD in six different H-2-compatible, multiple minor histocompatibility antigen-different murine strain combinations. In four of these combinations (C3H.SW----B6, DBA/2----B10.D2, B10.BR----CBA, and B10.S----SJL), lethal GVHD appeared to be caused almost entirely by Lyt-2+ cells; the injection of L3T4+ cells resulted in low mortality even when these cells were presensitized to the recipient antigens. In the remaining two combinations (B10.D2----DBA/2 and B10.D2----BALB/c), L3T4+ T cells were able to cause a high incidence of GVHD and were more potent than the Lyt-2+ cells. The implications of these findings are discussed.

Identification of interleukin 2-producing T helper cells within murine Lyt-2+ T lymphocytes: frequency, specificity and clonal segregation from Lyt-2+ precursors of cytotoxic T lymphocytes

The prime aim of this study was to assess whether the autonomous primary mixed lymphocyte culture response of Lyt-2+ T cells towards class I major histocompatibility complex (MHC) antigens reflects in terms of interleukin 2 (IL2) production and cytotoxicity the activation of multifunctional Lyt-2+ T cells, or the activation of functionally distinct T cell subsets. The results demonstrate that highly purified Lyt-2+ T cells proliferate in response to class I MHC antigens, as opposed to L3T4+ T cells which react towards class II MHC antigens. In both responder cell types proliferative responses are associated with IL2 secretion, while only Lyt-2+ T cells develop measurable cytotoxic effector cells. The precursor frequency of IL2-producing helper cells in MHC class I-reactive Lyt-2+ T cells equals that in MHC class II-reactive L3T4+ T cells (f = 1/500-1/1000). In clonal segregation analysis greater than 90% of Lyt-2+ colonies secreting IL2 do not develop cytotoxic activity, while greater than 90% of Lyt-2+ cytotoxic T cells fail to produce detectable IL2. A minority of less than 10% of Lyt-2+ T cells appears to be bifunctional. As such the results point out the existence of functionally committed T cells within class I MHC-reactive Lyt-2+ T cells able to produce either IL2 or to develop into cytotoxic effector cells.

Survey of experimental data on fetal liver transplantation

Fetal liver transplantation has been shown to induce hematological and immunological reconstitution in irradiated rodents, dogs, horses, and sheep. Engraftment and reconstitution without GvHD has been readily obtained using histocompatible donors. When mismatched fetal donors were used, a comparatively larger number of donor cells was required, in addition to pre-treatment of host with higher doses of irradiation or irradiation plus chemotherapy. Stem cell suspensions devoid of any T lymphocyte can be transplanted across major histocompatibility barrier without inducing overt GvHD. The transplanted animals become tolerant to both donor and host grafts.

Sustained recovery of hematopoiesis and immunity following transplantation of fetal liver cells in dogs

We studied the ability of fetal liver cells to reconstitute hematopoiesis and immunity in lethally irradiated dogs. Engraftment and sustained lymphoid and hematopoietic recovery was achieved when the recipients received a preparative regime of high-dose total body irradiation (TBI) alone followed by transplantation of DLA-identical fetal liver. The combination of high-dose TBI and cyclosporine allowed engraftment in DLA-mismatched fetal liver transplants. Typical features of graft-versus-host disease (GvHD) did not occur although autoimmune-like syndromes (myasthenia gravis, immune thrombocytopenia) were observed in some recipients. Hematopoietic recovery was rapid and complete. Recovery of T- and B-lymphocyte function was comparatively delayed, but sufficient to prevent opportunistic infections after the initial 3 months post transplant. These data indicate that cells from a single fetal liver can reconstitute hematopoiesis and immunity in DLA-mismatched recipients and suggest that human fetal liver cell transplantation may be an effective source of stem cells for patients who lack an HLA-identical donor for bone marrow transplantation.

T-cell depletion of allogeneic bone marrow prevents acceleration of graft-versus-host disease induced by exogenous interleukin 2

Highly purified human recombinant interleukin 2 (IL-2) markedly accelerated lethal GVHD in the H-2-identical B10.BR----CBA combination, but had no effect when the donor cells were depleted of mature (Thy-1.2-positive) T lymphocytes, indicating a strong immunopotentiating effect of IL-2 on mature T cells causing GVHD. In the same donor-host combination, IL-2 did not influence the recovery from the post-transplantation bone marrow aplasia. The results suggest that IL-2 could be considered for adjuvant hormonal therapy to enhance immune recovery in recipients of T-cell-depleted allogeneic marrow.

Functions of purified L3T4+ and Lyt-2+ cells in vitro and in vivo

The expression of L3T4 and Lyt-2 cell surface molecules separates T cells into two broad, non-overlapping subsets: typical T helper cells are L3T4+ Lyt-2- whereas most T killer cells and their precursors are L3T4- Lyt-2+. This review compares highly purified populations of unprimed L3T4+ and Lyt-2+ cells for their capacity to respond to class I vs. class II H-2 alloantigens. Various parameters are considered, including generation of mixed lymphocyte reactions (MLR) and cell mediated lympholysis (CML) in vitro, proliferative responses in irradiated mice, graft-versus-host reactions and skin allograft rejection. In all of these assays the two T cell subsets exhibit marked specificity in their response to H-2 alloantigens, L3T4+ cells responding only to class II and not class I differences and Lyt-2+ cells showing reciprocal specificity. Contrary to current dogma, the bulk of the evidence suggests that primary responses of Lyt-2+ cells do not depend on exogenous help provided by other T cells.

Gut injury in mouse graft-versus-host reaction. Study of its occurrence and mechanisms

The occurrence, nature, and pathogenesis of intestinal lesions were studied in a number of graft vs. host reaction (GVHR) conditions in mice, combining variations in the nature of the following: the F1 hosts (newborn or adult, normal or lethally irradiated), the injected parental T cells (mixed or selected subsets of Lyt2+ or L3T4+ cells), and the antigenic stimulus (semi-allogeneic or restricted to class I or II MHC loci). The following conclusions were drawn: Three gut alterations are always associated: donor T cell infiltration, predominating in the crypt region; acceleration of the epithelium renewal; and increased epithelial Ia expression. The initial event is T-cell infiltration, which results from stimulation within the Peyer patches followed by cyclic traffic, i.e., migration into the thoracic duct and then seeding to the whole gut mucosa. Both Lyt2+ and L3T4+ cells can infiltrate the gut wall, the extent of the infiltration by a given subset depending upon the capacity of the donor blasts to circulate in the thoracic duct (higher for L3T4+) and then to home in the gut (much higher for Lyt2+ blasts) and the nature of the alloantigenic stimulation that governs the extent of each donor subset proliferation. Both donor T-cell subsets can induce gut epithelial damage, but for a comparable amount of infiltrating cells, L3T4+ cells induce more lesions. When the antigenic stimulation is restricted to class I or class I MHC loci, gut GVHR is much more easily elicited across class II MHC differences, which stimulate preferentially L3T4+ donor cells. The main mechanism of epithelial damage is not direct cytotoxicity, but more probably lymphokine(s) release.

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)

Properties of purified T cell subsets. I. In vitro responses to class I vs. class II H-2 alloantigens

In light of the widely accepted view that Ia-restricted L3T4+ T helper cells play a decisive role in controlling the differentiation of Lyt-2+ cells, experiments were designed to examine whether Lyt-2+ cells can respond to antigen in the absence of L3T4+ cells. The results showed that highly purified Lyt-2+ cells gave high primary mixed lymphocyte reactions (MLR) to various class I differences, including both mutant and allelic differences; responses to class II (Ia) differences were generally undetectable with Lyt-2+ cells. The intensity of MLR to class I differences was not affected by addition of anti-L3T4 monoclonal antibodies (mAb) to the cultures or by removing T cells from the stimulator populations. Negative selection experiments showed that Lyt-2+ cells could respond to class I differences across Ia barriers. MLR of purified Lyt-2+ cells peaked on days 3-4 and then fell sharply; background responses with syngeneic stimulators (auto-MLR) were virtually absent. Parallel experiments with purified L3T4+ cells showed that this subset responded in MLR only to class II (Ia) and not class I differences, reached peak responses only on day 6 rather than days 3-4, and often gave high auto-MLR. Within the first 3-4 d of culture, MLR were generally higher with Lyt-2+ cells than L3T4+ cells. Although no evidence could be found that Ia-restricted L3T4+ cells were required for the response of Lyt-2+ cells, presentation of antigen by Ia+ cells appeared to be essential. Thus, responses were ablated by pretreating stimulator cells with anti-Ia mAb plus C'. Significantly the failure of Lyt-2+ cells to respond to anti-Ia plus C'-treated stimulators could not be restored by adding syngeneic spleen cells; addition of IL-2 led to only a minor (15%) restoration of the response. It is suggested that Ia+ cells provide an obligatory second signal required by Lyt-2+ cells.

Studies addressing the mechanism of anti-asialo GM1 prevention of graft-versus-host disease due to minor histocompatibility antigenic differences

In mice, as in humans, lethal graft-versus-host disease (GVHD) with skin involvement often occurs in immunoincompetent recipients of donor hematopoietic cells in spite of matching at major histocompatibility loci and nonreactivity in mixed lymphocyte culture, if donor and recipient are disparate at several minor histocompatibility loci. In mice, both death and skin disease can be prevented by the use of an antiserum containing antibodies to a cell surface glycolipid, asialo GM1 (ASGM1). Because treatment of only the recipients with anti-asialo GM1 substantially reduces the subsequent proliferation of infused donor lymphoid cells, we infer that anti-asialo GM1 interferes with a host minor-antigen-presenting cell, so that donor lymphocytes fail to see minor host antigens as immunogenic. Of the tissues examined by immunofluorescence microscopy, ASGM1 was found on the epidermal Thy-1+ dendritic cell, on dendritic cells in the thymus, and as has been previously described, on lung and spleen cells. Following the intravenous administration of anti-asialo GM1, only the spleen showed an obvious change, losing approximately 80% of its ASGM1 + cells. Further analysis of spleen cells bearing ASGM1 may better define the phenotype of the inferred minor antigen-presenting cell and lead to a method of improving the outcome of human bone marrow transplantation.

Graft-vs.-host reactions: mechanisms and contemporary theories

A graft-vs.-host (GvH) reaction can be initiated by injection of immunocompetent lymphocytes into a histoincompatible host that is unable to reject these cells. The reaction is characterized by splenomegaly, hepatomegaly, lymph node atrophy, body weight loss, dermatitis, and diarrhea, often leading to mortality. The onset and severity of the GvH reaction are determined by differences in histocompatibility antigens between the donor and the acceptor, and by the number and nature of the transplanted allogeneic cells. Many different in vivo and in vitro systems have been devised for experimental studies of the GvH reaction. In several of these models, however, different parameters are measured. Furthermore, the conclusions drawn from these investigations sometimes contradict each other. This paper reviews the experimental data, and discusses the mechanisms underlying the GvH reaction.

Restricted recognition of H-2 subregion coded alloantigens in delayed-type hypersensitivity

Subcutaneous (sc) immunization of mice with H-2K, I, or D incompatible spleen cells induces a state of host-versus-graft (HvG) delayed-type hypersensitivity (DTH). The DTH reaction is elicited by challenging the immunized mice in a hind foot with similar allogeneic spleen cells and is measured as the subsequent foot swelling. DTH effector T cells specific for H-2I-coded alloantigens, but not for H-2K/D-coded alloantigens, can be induced in a graft-versus-host (GvH) model as well. In this paper we report that under HvG as well as under GvH conditions the recognition of class II antigens by DTH effector T cells is restricted by class I molecules. Furthermore, DTH effector T cells induced by sc immunization with class I antigens appear to be restricted by class II molecules.

Cellular basis of graft-versus-host reactions

The biologic basis of Graft-Versus-Host Disease (GVHD) is presented as an extremely complex immunopathologic syndrome that involves interaction between many different donor and host cell types. A model of acute lethal GVHD was employed where adult unirradiated (DA X LEW)F1 rats were injected with LEW spleen and lymph node cells. Controls received the same dose of syngeneic cells. At intervals from 2 to 21 days after cell injection, GVHD and control animals were killed and nonadherent cell suspensions prepared from their lymph nodes, spleen and peripheral blood. Cell suspensions were treated with LEW-anti-DA-alloantiserum or normal LEW serum and then analyzed for sIgM+ (B cells), W 3/13+ (T cells), and IgG-Fc receptors (FcR). Evidence is discussed for the selective removal of host cells with the alloantiserum. In addition, the level of naturally cytolytic (NK/NC) cells was assessed by adding GVHD and control nonadherent lymphoid cells to heterologous lymphoma and sarcoma target cells. Evidence is presented that during acute GVHD, in this parental----F1 combination, there is an early increase within most compartments of donor as well as host W 3/13+ and W 3/13+FcR+ cells. NK/NC cells are increased as well at day 7. During middle stages of acute GVHD, host sIgM+ cells predominate. Late-stage acute GVHD rats contain few donor and host W 3/13+, W 3/13+FcR+, and NK/NC cells but many null cells most of which are FcR-. The importance of unraveling the nature of donor- and host-cell interactions occurring during acute GVHD, which result in rats whose lymphoid tissues are severely depleted of all nonadherent lymphoid cells but FcR- null cells, is discussed.

The localized primary cytotoxic T-cell response to cells expressing minor histocompatibility differences

Cytotoxic T lymphocytes (CTL) are able to eliminate P815 (DBA/2) mastocytoma cells growing in cerebrospinal fluid of BALB/c H-2-compatible but minor histocompatibility (H) antigen-different mice and in H-2-incompatible C3H/He mice. We examined the magnitude of the primary CTL response to multiple, minor H antigens and to determinants of the major histocompatibility complex (MHC) by using a direct cytolytic assay and limiting-dilution analysis to estimate CTL frequency. By these criteria, no obvious differences emerged, and the responses appeared comparable at the site of inflammatory process, despite differences in the number of clonal progenitors. Experiments with radiation chimeras showed evidence of a strong cytotoxic T-cell response against P815 cells in [(ddd X bbb)F1----ddd] and (F1----bbd), but not in (F1----bbb) radiation chimeras. Therefore, this cytotoxic T-cell response against minor H antigens obeys the postulated rules for thymic restriction of precursors. Compatibility at the H-2 D-end of the MHC is apparently sufficient to ensure a strong response.

Allosuppressor- and allohelper-T cells in acute and chronic graft-vs.-host (GVH) disease. III. Different Lyt subsets of donor T cells induce different pathological syndromes

Previous work from this laboratory has led to the hypothesis that the stimulatory pathological symptoms of chronic graft-vs.-host disease (GVHD) are caused by alloreactive donor T helper (TH) cells, whereas the suppressive pathological symptoms of acute GVHD are caused by alloreactive T suppressor (TS) cells of the donor. In the present paper we analyzed the Lyt phenotypes of B10 donor T cells required for the induction of either acute or chronic GVHD in H-2-different (B10 X DBA/2)F1 recipients. First, nonirradiated F1 mice were used as the recipients. We found that unseparated B10 T cells induced only a moderate formation of systemic lupus erythematosus (SLE)-like autoantibodies, but a high percentage of lethal GVHD (LGVHD). In contrast, Lyt-1+2- donor T cells were unable to induce LGVHD in these recipients; these cells were capable, however, of inducing a vigorous formation of SLE-like autoantibodies and the formation of severe immune-complex glomerulonephritis. Lyt-1-2+ T cells were incapable of inducing either acute or chronic GVHD. In another experiment, the sensitivity and accuracy of the GVH system were increased by using irradiated F1 mice as recipients and by comparing donor-cell inocula that contained similar numbers of T lymphocytes. In addition, donor-cell inocula were used that had been tested for their allohelper and allosuppressor effects on F1 B cells in vitro. In the irradiated F1 recipients, too, unseparated donor T cells were superior to T cell subsets in inducing LGVHD; Lyt-1-2+ donor cells were completely and Lyt-1+2- donor cells were almost incapable of doing so. In contrast, Lyt-1+2- T cells, but neither unseparated T cells nor Lyt-1-2+ T cells, were capable of inducing a vigorous formation of SLE-like auto-antibodies. We conclude that the stimulatory pathological symptoms of chronic GVHD are caused by Lyt-1+2- allohelper T cells. In contrast, the development of the suppressive pathological symptoms of acute GVHD appears to involve alloreactive Lyt-1+2+ T suppressor cells.

Selection of the T-cell repertoire during ontogeny

This report examines conflicting hypotheses concerning T-cell repertoire selection in terms of H-2 restriction during ontogeny. The experiments described in this report were incompatible with the hypothesis that bias in the repertoire is solely a consequence of "more or less intentional priming" by foreign antigen. Rather, results indicate that the repertoire is selected by self major histocompatibility complex (MHC) antigens in the absence of foreign antigens. Allorestricted T cells, the existence of which was previously thought to be incompatible with the concept of complete repertoire selection by self MHC antigens, were shown to significantly cross-react on targets expressing self MHC antigens. Thus, it is possible that allorestricted T cells are simply cross-reactive T-cell clones restricted by self MHC antigens; indeed, all experimental data were compatible with the idea of complete selection of the T-cell repertoire in terms of H-2 restriction by self MHC antigens.

Induction of cytotoxic T cell precursors in vivo. Role of T helper cells

Strain AS rats respond with two populations of cytotoxic T lymphocytes to stimulation in vitro by the major histocompatibility complex (MHC)-incompatible strain HL rat tumor (HL-A2T2). One is specific for MHC alloantigens present on both HL-A2T2 and normal HL targets, the other is tumor specific. The activation of these killer cells requires helper T lymphocytes. The tumor-specific helper cells depend on syngeneic radioresistant accessory cells to present the tumor antigens in an immunogenic form. The appropriate helper-accessory cell interaction results in the production of soluble factors which then induce the maturation of precursor cells into effective killer cells. Studies with a procedure for inducing negative selection of T cells in vivo showed that short-term exposure to HL-A2T2 tumor induced selection only for TH but not cytotoxic T lymphocyte precursors (CTLp). Simultaneous injection of supernatants from concanavalin A-activated spleen cell cultures, however, did produce selection of CTLp. These and other findings suggest that under normal circumstances in vivo, both signals (recognition of antigen and acceptance of maturation factors) are provided in the vicinity of an antigen presenting macrophage-like accessory cell.

Lethal GVHD across minor histocompatibility barriers: nature of the effector cells and role of the H-2 complex

Transfer of T-cells to heavily irradiated, H-2-compatible mice frequently leads to a high incidence of lethal graft-versus-host disease (GVHD). Lymphoid cells depleted of Thy1+ cells fail to cause GVHD. Studies with a variety of different, H-2-compatible, strain-combinations suggest that minor, histocompatibility antigens (minor HA) are the main targets for eliciting lethal GVHD. Experiments in which T-cells are negatively selected to minor HA by acute blood-to-lymph recirculation through irradiated hosts have indicated that the T-cells eliciting GVHD to minor HA, are H-2-restricted. In H-2-compatible hosts, the donor T-cells recognize the minor HA of the host and become temporarily trapped in the lymphoid tissues for 1-2 days; during this stage of negative selection, the donor T-cells entering the lymph are specifically devoid of cells able to elicit GVHD against the host, minor HA on further transfer. When the selection host is H-2-different with respect to the donor T-cells, by contrast, the T-cells ignore the host, minor HA and negative selection fails to occur. The T-cells recirculate normally and are unimpaired in their capacity to elicit GVHD on further transfer. By the use of various H-2-recombinant mice as selection hosts it has been shown that, as for T-cells exerting cell-mediated lympholysis (CML) to minor HA in vitro, the T-cells which elicit lethal GVHD to minor HA comprise two distinct subsets of H-2-restricted cells. One subset recognizes minor HA in the context of H-2K (or K end) molecules whereas the other is specific for minor HA-plus-H-2D. Curiously, in marked contrast to the findings on CML responses in vitro, no evidence has been found that H-2I-restricted T-cells contribute to GVHD, either as effector cells or as helper cells. Purified populations of Lyt 1-2+ T-cells have potent GVHD activity, whereas Lyt 1+2- cells fail to cause GVHD. Studies with various types of bone-marrow chimeras suggest that in the induction phase, T-cells recognize minor HA only on lymphohematopoietic cells. In the effector phase, by contrast, non-marrow-derived cells appear to be the main targets of attack. Although the pathogenesis of GVHD is poorly understood, the lethal form of the disease probably reflects the penetration of mucosal surfaces by pathogenic organisms, perhaps as the result of direct destruction of epithelial cells by minor HA-specific cytotoxic lymphocytes. Direct support for this notion has yet to be obtained.

Are increased IgE-levels a signal of an acute graft-versus-host reaction?

IgE-levels are markedly elevated following bone-marrow transplantation in patients with and without GVHD. In patients with GVHD, there is a significant correlation between the timing of the IgE-increase and the appearance of clinical GVHD (p less than 0.01). The highest IgE-level (8000 kU/l) was noted in a recipient of a syngeneic graft. During the IgE-peak, the serum from this patient contained low concentrations of IgE reacting with several tested allergens as well as for the hapten TNP, which indicated polyclonal activation. In a patient with a known allergy to animal danders, RAST tests were positive against dog and cat both before and six weeks after total body irradiation and transplantation with marrow from a non-allergic donor. A slight increase in the amount of allergen-specific, IgE-antibodies was seen during the increase in total IgE. A non-allergic patient was transplanted with marrow from a donor allergic to timothy. Timothy-specific, IgE-antibodies were detected immediately after transplantation but they disappeared within a few days and could not be detected during the period of increase in total IgE. We believe that the IgE-elevation seen after conditioning with cytotoxic drugs and total body irradiation in BMT-patients is a polyclonal response in host B-cells induced during an acute, GVHR and influenced by disturbed regulatory T-cells. Lymphocytes from patients with acute GVHD had unusually large numbers of IgG/PFC in vitro after stimulation with staph. aureus Cowan 1 (p less than 0.001), which may reflect a clonal expansion of responsive B-cells.

H-2-restricted graft-versus-host reaction: foreign determinants and restriction elements

Nylon-wool-purified T cells from radiation chimeras cause a lethal graft-versus-host reaction (GVHR) in irradiated, bone-marrow-protected recipients only if the recipient shares a restriction element with the T-cell donor and also expresses antigens foreign to the donor. Class I molecules (H-2K and H-2D) can act as restriction elements, but restriction to class II molecules could not be demonstrated. However, class II molecules as well as H-2K and some non-H-2 determinants could serve as foreign antigens.