Redundant Mechanisms for Dendritic Cell Activation in GVHD Induction: Signalings Via TLRs, TNF-α, IL-1 and CD40 Are Not Required

Graft-versus-host disease (GVHD) is initiated by antigen-presenting cells (APCs) that prime alloreactive donor T cells. Amongst APCs, dendritic cells (DCs) are the most potent at priming naïve T cells. A paradigm of DC function holds that instructive maturation signals transform “immature” DCs into “mature” DCs which are optimized for T cell activation. Signaling via Toll-like receptors (TLRs) has emerged as a central mechanism for inducing this maturation in anti-pathogen responses. However, in allogeneic stem cell transplantation, there is no specific pathogen and the signals that induce DC maturation in GVH responses have not been defined. To study potential maturation stimuli we performed GVHD experiments in model systems wherein host APCs are essential for GVHD induction (C3H.SW (H-2^b)→B6 and B6^bm12→B6). We then compared GVHD in wild type (wt) recipients to that in hosts genetically unable to respond to various DC maturation signals. We first used as hosts mice deficient in TLR4, CD40, IL1-R or TNFR1/R2 and found that in C3H.SW→B6 model, GVHD in each of these was similar. Likewise, GVHD was similar in wt and mice deficient in MyD88, essential for signaling via all TLRs except TLR3 and TLR4. To focus our studies on APCs, we used as hosts radiation bone marrow (BM) chimeras in which wt B6 (CD45.1) mice were reconstituted with BM from CD45.2 wt, MyD88^−/−, TRIF^−/− (required for signaling via TLR3 and, in-part, TLR4), and MyD88/TRIF^−/− mice, in which signaling by all TLRs, IL-1 and IL-18 is completely blocked. When retransplanted with C3H.SW BM and CD8 cells, all chimeras developed similar clinical and histologic GVHD. To exclude that T cells were being primed by residual wt host APCs in BM chimeric recipients, we used as hosts B6^bm12 mice that were reconstituted with BM from B6 wt, MyD88^−/−, TRIF^−/− or MyD88/TRIF^−/− mice. These chimeras were then retransplanted with B6^bm12 BM and CD4 cells. Because residual host APCs in these BM chimeric hosts were B6^bm12 and therefore syngeneic with the donor cells, T cell priming could only occur on B6 background APCs derived from the first transplant. As in the prior experiments, all chimeras developed similar clinical GVHD; pathology scoring is underway. Therefore in contrast to nearly all adaptive T cell responses, GVHD induction in both MHC-matched and MHC-mismatched systems did not require APC maturation signals mediated by TLRs. Absent any phenotype when APC maturation signals were blocked, we used the same approach in the C3H.SW→B6 model to study the roles of IL12 and type I IFNs, which are produced as a consequence of DC maturation. B6 p35^−/− and B6 wt mice developed similar GVHD, excluding an essential role for host IL12. Type I IFN receptor-deficient (IFNIR^−/−) mice developed similar GVHD as did wt controls, except that IFNIR^−/− CD8 recipients lost more weight but had less histologic liver GVHD (P=0.0024). To focus on the activity of Type I IFNs on host hematopoietic cells, we used as recipients IFNIR^−/− → B6 chimeras. Both clinical and histologic GVHD were similar in IFNIR^−/−→B6 and wt→B6 control chimeras, suggesting that the differences in weight loss and hepatic GVHD observed in IFNIR^−/− hosts, were due to parenchymal expression of IFNIR^−/−. In sum, these data indicate that DC maturation signals in GVHD are at a minimum redundant, and independent of TLR ligands such as LPS, which are certainly available to the host immune system. This may be a consequence of the ubiquitous expression of alloantigen, such that any maturation signal will create APCs primed to activate donor T cells. If so, strategies for preventing GVHD by blocking APC maturation may need to target final common APC maturation pathways. Alternatively, instructive APC maturation may not be required for alloreactive T cell activation in GVHD.