Cross-Presentation by Host CD8α⁺dendritic Cells Augments Graft-Versus-Tumor Effect without Aggravating Graft-Versus-Host Disease After Allogeneic BMT

Abstract 310

Host hematopoietic derived antigen presenting cells (APCs) play an important role in the induction of GVHD and GVT. While all of the host hematopoietic derived APCs express and directly present allo-antigens that drive GVHD and GVT responses, the tumor specific antigens (TSA) expressed only on the tumor cells but not on the host professional APCs. TSAs will therefore need to be cross-presented on host hematopoietic derived APCs to generate optimal tumor antigen specific GVT response. We therefore reasoned that absence of cross-presentation by host hematopoietic derived professional APCs will not affect allo-antigen driven GVHD severity but may reduce GVT driven by TSAs. Because cross-presentation of antigens is mediated primarily by CD8α⁺dendritic cell (DCs) subset, we hypothesized that that cross-presentation by host CD8α⁺DCs will be critical for optimal GVT without aggravating GVHD. We utilized wild type (WT) B6 and B6 Batf3^−/− animals (that lack the CD8α⁺ DCs) as recipients in well characterized MHC matched, minor mismatched C3H.SW “type=”#_x0000_t75”>B6 model of acute GVHD. WT and Batf3^−/− animals received 10 Gy and were transplanted with 5 × 10⁵ purified splenic CD8⁺T cells and 5 × 10⁶ BM from either syngeneic B6 or allogeneic C3H.SW donors. Consistent with the hypothesis, the WT and the Batf3^−/− animals showed similar survival and clinical severity of GVHD (P = NS). The increase in mortality was associated with a significant increase in the histopathological damage of the GVHD target organs (liver and intestine).

Next, to determine the effect of host CD8α DCs on GVT responses, WT and Batf3^−/−mice were conditioned and transplanted as above with syngeneic B6 or the C3H.sw donors along with 2×10⁴MBL-2 tumor cells (which syngeneic to the host and express TSAs in addition to the allo-antigens that can be recognized by the donor T cells). All of the allogeneic WT and Batf3^−/− mice developed GVHD. However, while none (0%) of the WT B6 animals died with tumor, 80% of the Batf3 KO mice had tumors at death (P<0.01, see Table). When the tumor dose was increased over two folds, 40% of the allogeneic WT animals died with tumor while 100% of the Batf3 KO animals died with tumors (P<0.05, see Table). Similar results were obtained with different tumor (EL-4) and GVHD (BALB/c→B6) models ruling tumor and strain dependent artifacts.

Because toll-like receptor 3 (TLR-3) is primarily, but not exclusively, expressed on CD8α⁺DC subsets and has also been implicated in cross-presentation, to further confirm the relevance of cross-presentation by host APC subsets in mediating GVT, we generated [WT-B6→B6Ly5.2] and [TLR3^−/−B6→B6Ly5.2] chimeras and utilized them as allo-recipients along with or without 5×10⁵ MBL-2 tumor cells. All of the allogeneic [WT-B6→B6Ly5.2] and [TLR3^−/−B6→B6Ly5.2] animals demonstrated similar GVHD. However, while only 50% of allogeneic [WT-B6→B6 Ly5.2] animals died with tumor, 100% of allogeneic [TLR3^−/−→B6Ly5.2] animals died with tumor (P<0.05). To further confirm the specificity of TLR-3 and to determine whether GVT effect can be augmented by enhancing cross-presentation, we treated the chimeras with either poly I:C (which is a TLR-3 specific ligand and shown to promote cross-presentation) at 50μg/mice (day 0 and 1) or diluent control along with the MBL2-tumors after allo-BMT. Treatment with poly I:C reduced tumor induced mortality in the allogeneic [WT-B6→B6 Ly5.2] animals (50% vs. 8%, P<0.01, see Table), but did not alter tumor induced death in the [TLR3^−/−→B6 Ly5.2] animals (100% vs. 93%, P=NS). Collectively, these data demonstrate, for the first time to our knowledge, that modulating antigen presentation on host APC (CD8α⁺ DC) subsets can augment GVT without aggravating GVHD.