Complement Component C3 Mediates Th1/Th17 Polarization in Human T Cell Activation and Cutaneous Graft-Versus-Host Disease

Graft-verses-host disease (GVHD) is a major complication in allogeneic bone marrow transplantation (allo-HSCT), and characterized by epithelial cell injury in skin, intestine and liver.  The development of GVHD involves donor T cell activation including proliferation, differentiation and inflammatory cytokine production, which lead to specific tissue damage. The interactions between the complement system and lymphocytes have been shown to regulate alloreactive T cell and APC function in the setting of allograft rejection. There are three pathways that activate the complement system: the alternative, lectin and classical pathways; all of which converge on the formation of the C3 convertase to propagate the complement cascade. Recently, we reported that mice deficient in the central component of complement system C3 had significantly lower GVHD-related mortality/morbidity and C3 modulated Th1/Th17 polarization in mouse GVHD. Given the emerging role of complement in alloimmune responses and T cell activation in animal models, it is important to address whether C3 modulates human T cell activation, polarization, expansion and differentiation. Compstatin is a 13-residue cyclic peptide that specifically binds to human C3 and inhibits complement activation, thus a favorable precursor peptide for the development of an anti-complement drug for oral use. Herein, we investigate the functional consequences of blocking C3 activation on human T cell activation. The production of IFN-γ (Th1), IL-4 (Th2), IL-17 (Th17), IL-2 and TNF-α was determined simultaneously in normal donor samples to examine whether Compstatin affects T cell activation and polarization in vitro. We found that blocking C3 activation with Compstatin significantly inhibits Th1/Th17 polarization in activated human CD4⁺ T cells. The production of IL-2 and TNF-α are reduced in CD4⁺ but not in CD8⁺ T cells. Moreover, Compstatin treatment significantly decreases the proliferation of both CD4⁺ and CD8 ⁺T cells stimulated with OKT3 plus CD28 or allogeneic DCs in MLR. It has been reported that patients with sclerotic-type chronic GVHD (ScGVHD) have significantly elevated C3 in the serum. We examined the degree of C3 deposition in the skin and lip samples retrieved from our GVHD tissue repository of human allo-HSCT recipients. In the skin GVHD tissues, C3 depositions are found in the squamous epithelium and dermis, blood vessels and damaged sweat glands. In the lip biopsy of GVHD patients, C3 depositions are found in the lesions associated with gland damage and regeneration, and damaged blood vessels. In summary, we conclude that C3 mediates Th1/Th17 polarization in human T cell activation and skin GVHD in patients. Studies on complement system and GVHD will not only significantly advance our knowledge of GVHD but also provide a rationale for using complement inhibitors as novel therapeutic interventions for GVHD.