Ubiquitary Expressed Major Histocompatibility (MHC) Class I and Minor Histocompatibility Alloantigens (mHAg) Are No Relevant Targets For Graft-Versus-Tumor (GvT) Reactions In Mice After Allogeneic Bone Marrow Transplantation

Cell mediated GvT effects after allogeneic stem cell transplantation are heterogeneous with respect to effector cell populations, target antigens and their interrelation with graft-versus-host disease (GvHD). Mutated tumor-associated antigens (TAAs), as well as selectively or aberrantly expressed nonmutated antigens, represent potential targets for T cell-mediated GvT effects that are in principle separable from the generalized graft-versus-host reaction targeting widely expressed MHC alloantigens and mHAg. So far, it remains a matter of debate whether alloantigens ubiquitary expressed on tumor cells and normal host tissue are relevant targets of GvT responses.

To evaluate the role of MHC class I molecules and mHAg as targets for GvT reactions, we employed three different allogeneic parent-into-F1 murine transplant models (BALB/c [H-2d] or C57BL/6 [H-2b] → F1[BALB/c x C57BL/6] [H-2b/d] and BALB/c → F1[BALB/c x CBA/J] [H-2d/k]). As previously shown, these transplant models allow for separate variation in the histocompatibility between donor cells and normal host tissue on the one hand and between donor cells and tumor tissue on the other hand. Test tumors were the donor derived myeloma / leukemia cell lines MPC11 [H-2d] and C1498 [H-2b], which were stably transfected with MHC class I molecules H-2Kb, H-2Kd, H-2Kk or with the mHAg UTY.

Compared with non-GvHD controls (F1[H-2b/d] mice transplanted with allogeneic bone marrow cells from BALB/c [H-2d] alone), significant GvT effects against the BALB/c derived tumor MPC11 occurred in F1[H-2b/d] recipients with severe GvHD (transplanted with bone marrow and additional splenic lymphocytes from BALB/c). Identical histocompatibility antigens of donor and tumor cells precluded allorecognition of tumor cells, leaving TAAs the only possible target antigens in this particular setting, with GvHD as a driving force for augmentation of tumor specific immune responses. The MPC11 myeloma [H-2d], stably transfected with the C57BL/6 derived MHC class I molecule H-2Kb (MPC11-Kb), showed similar tumor growth compared to the wild type MPC11 in F1[H-2b/d] mice transplanted from BALB/c donors. Similar results could be observed with the C57BL/6 derived C1498 leukemia [H-2b], transfected with the BALB/c MHC class I alloantigen H2Kd, in F1[H-2b/d] mice transplanted from C57BL/6 donors. In both experimental settings, the artificially expressed alloantigens H2Kb and H2Kd were also expressed ubiquitarily on tissues of F1[H-2b/d] recipients. In sharp contrast, MPC11 cells transfected with the CBA/J derived H2Kk (MPC11-Kk) alloantigen showed a significantly reduced tumor growth in F1[H-2b/d] recipients transplanted from BALB/c donors, demonstrating that MHC class I alloantigens are relevant targets, when expressed on tumors but not on healthy recipient tissue. Inoculation of MPC11-Kk in F1[H-2d/k] mice 3 or 7 days after transplantation from BALB/c donors showed a trend towards a reduced tumor growth compared to wild type MPC11 in recipients with severe GvHD. Later application of tumor cells (14 days after transplantation) showed similar growth pattern of both tumors, suggesting that MHC class I antigens might be GvT targets in the early phase after allogeneic transplantation. To evaluate the impact of ubiquitarily expressed mHAg, we established a C1498 cell line, which stably expressed UTY (C1498-UTY), a known immunogenic antigen of the male HY gene. In comparison with the wild type control, C1498-UTY showed similar tumor growth in male or female F1[H-2b/d] recipients, transplanted from naive female C57BL/6 donors. Immunization of female C57BL/6 donors with male splenocytes resulted in a significantly reduced tumor growth of the C1498-UTY in female F1[H-2b/d] recipients. In contrast, tumor growth of the UTY expressing and wild type C1498 was similar in male F1[H-2b/d] mice transplanted from immunized female C57BL/6 donors.

Our experimental data indicate that a biologically relevant GvT effect targeting MHC class I or mHAgs mainly occurs when these alloantigens are not ubiquitarily expressed, even in the context of severe GvHD. Consequently, TAAs and tissue- /hematopoietic specific antigens are most likely the main targets of GvT effects observed after allogeneic stem cell transplantation.

No relevant conflicts of interest to declare.