Robert Korngold

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.

GRAFT-VERSUS-MYELOID LEUKEMIA RESPONSES FOLLOWING SYNGENEIC AND ALLOGENEIC BONE MARROW TRANSPLANTATION

A model for investigating graft-versus-leukemia (GVL) activity following syngeneic and MHC-compatible allogeneic BMT has been developed in C57BL/6 (B6) mice with use of the c-myc retrovirus-transformed MMB3.19 myeloid leukemia line. The MMB3.19 line was derived from a B6 mouse and expresses monocyte/macrophage markers, including Mac-1, Mac-2, F4/80, and LFA-1, in addition to H-2 class I and class II molecules. A challenge dosage of 10(5) of these leukemia cells was found to be completely lethal when injected into irradiated (850 cGy) B6 recipients, 1 day after the transplantation of syngeneic donor T cell-depleted-bone marrow. The addition of T lymphocytes to the donor inoculum prolonged recipient survival, and both CD4+ and CD8+ subsets were found to be capable of mediating this GVL activity. For the MHC-compatible allogeneic model, the C3H.SW-->B6 (850 cGy) strain combination was utilized, in which CD8+ T cells are known to cause graft-versus-host disease directed to minor histocompatibility antigens expressed by the recipient. In this case, both CD(4+)- and CD(8+)-enriched T cells were found to be capable of mediating GVL activity to MMB3.19 challenge, particularly if donor mice were presensitized with leukemia cells. Of most significance, only the donor CD4+ T cells mediated a GVL effect without the apparent induction of graft-versus-host disease.

Graft-Versus-Host Disease in Experimental Allogeneic Bone Marrow Transplantation

Summary Considerable progress has been made in defining the relative contributions of CD4+ and CD8+ cells to GVHD. Studies in mice have shown that, in isolation, each T cell subset is able to induce lethal GVHD in irradiated hosts. For hosts differing at the MHC (H-2), CD4+ cells cause GVHD directed to H-2 class II antigens whereas CD8+ cells produce GVHD to H-2 class I antigens. With H-2-matched hosts expressing multiple minor H antigens, induction of lethal GVHD is largely under the control of CD8+ cells. Which particular minor H antigens provide the targets for GVHD in mice is still unclear. Clarifying this question is complicated by the finding that the target antigens for GVHD do not necessarily correlate with the targets for cytotoxicity measured in vitro; moreover, immunodominance occurs when T cells are exposed to multiple minor H antigens in vivo. In terms of clinical application, there is a need to devise animal models for improving the success of HLA-matched bone marrow transplantation. Selectively depleting the marrow inoculum of CD8+ cells and preimmunizing the donor against viral pathogens are two procedures which are currently under study.

T cell recognition of antigen in vivo: Role of the H-2 complex

Gene products of the major histocompatibility complex (MHC) play a decisive role in controlling a variety of different T cell functions, including cell-mediated lympholysis (CML) [165], delayed-type hypersensitivity (DTH) [79], proliferative responses to antigen-pulsed macrophages [103], and T-B collaboration [60]. The available evidence suggests that T cell specificity is not directed to free antigen but to antigen associated with MHC determinants. Despite intensive speculation, it remains unclear whether T cells recognize the association of antigen plus MHC

H-2-Restriction of T Cells Mediating Lethal Graft-Versus-Host-Disease to Minor Histocompatibility Determinants

Although the etiology and pathogenesis of graft-versus-host disease (GVHD) continues to excite controversy and speculation, it is now generally accepted that the disease reflects alloaggres-sion mediated by mature post-thymic T cells (1–3). The pioneering studies of Siraonsen (1) established that the most florid forms of GVHD occur when the donor and host differ at the major histocompatibility complex (MHC). In recent years, however, it has become clear that severe and often fatal forms of GVHD occur in MHC-com-patible combinations, e.g. following HLA-matched bone marrow transplantation in man (3). Although the etiology of this form of GVHD is still debated, our own studies in mice suggest that responses to minor histocompatibility antigens (minor HA)4 are the principal cause of the disease, at least in this species (4).