Novel Therapeutic Approach To Graft-Versus-Host Disease With Allele-Specific Anti-HLA Monoclonal Antibody

Allogeneic hematopoietic stem cell transplantation (allo-HSCT) using cord blood or haploidentical donor is a promising alternative option for patients who can not find an HLA-matched donor. However, HLA-mismatch allo-HSCT may be complicated by graft-versus-host disease (GVHD), a major cause of non-relapse mortality mediated by alloreactive T cells.

Infusion of anti-thymocyte globulin (ATG) is used both as a treatment for and as a prophylactic against GVHD but ATG, like other immunosuppressive regimens, reacts against cells without distinguishing between donor and recipient cells. ATG is polyclonal and causes many side effects such as allergic reaction and increasing susceptibility to infection due to repression of T cells. Our ultimate goal is to develop a novel therapeutic approach to GVHD with anti-HLA monoclonal antibody specifically targeting the mismatched HLA molecule of donor origin to transiently ablate alloreactive T cells.

To generate allele-specific anti-HLA antibody (ASHmAb), we established a rapid and efficient strategy using two different HLA-transgenic mouse strains, HLA-A24 and HLA-A2 and immunized them with HLA-A* 02:01 and HLA-A* 24:02 tetramers loaded with cytomegalovirus PP65 peptide, respectively. Antibodies were screened for HLA allele-specific complement-dependent cytotoxicity in vitro. We successfully established three killing ASHmAbs (kASHmAbs) against HLA-A* 02:01, -A* 02:01/–A* 03:01, and -A* 23:01/-A* 24:02.

In vivo cytotoxicity and specificity of a kASHmAb against HLA-A* 02:01 (A* 02:01-kASHmAb) was examined in detail using a xenogeneic GVHD mouse model. To induce fatal GVHD, non-irradiated NOD/Shi-scid/IL-2Rγ^null (NOG) mice were injected with either HLA-A2 or HLA-A26 positive human peripheral blood mononuclear cells (PBMCs) from healthy donors (i.e. HLA-A2 mice or HLA-A26 mice, respectively). Administration of A* 02:01-kASHmAb promoted the survival of HLA-A2 mice to 100%, with a mean survival of more than 6 months. In contrast, administration of A* 02:01-kASHmAb to nonreactive HLA was not effective; all HLA-A26 mice died within 2 months (p<0.0001), demonstrating kASHmAb can successfully treat a GVHD mouse model.

Next, we examined the cytotoxicity of the kASHmAb on the human graft in bone marrow using humanized NOG mice. Sublethally irradiated NOG mice were reconstituted with CD34 positive hematopoietic stem/progenitor cells from HLA-A2 positive cord blood. These humanized mice showed about 70-90% human T, B and myeloid cells in the peripheral blood. When these mice were administrated with A* 02:01-kASHmAb for five consecutive days (60 mg/day), mature human blood cells disappeared immediately from the mouse peripheral blood. Interestingly, human PBMCs were detectable again in the mouse blood in 2-4 weeks after the last kASHmAb administration, suggesting kASHmAb may have preferential cytotoxic effect on mature PBMCs sparing hematopoietic stem progenitor cells. The mechanism of this preferential killing is not clear, but an optimal dose of kASHmAb may be safely administered to GVHD patients without permanently ablating the graft and necessitating repeated transplantation.

This study is the first reported instance of using the cytotoxic effect of HLA antibodies against GVHD. Unlike ATG, kASHmAb specifically killed donor cells and contributed to the survival of the recipients. While this novel therapeutic approach requires a panel of kASHmAbs to different HLA alleles, this may open a new door for treatment of severe GVHD. Furthermore, optimization of the kASHmAb administration protocol may contribute to tolerance induction.