Natural suppressor-like cells in local graft-vs-host disease

Recipient NK cell inactivation and intestinal barrier loss are required for MHC-matched graft-versus-host disease

Previous studies have shown a correlation between pretransplant conditioning intensity, intestinal barrier loss, and graft-versus-host disease (GVHD) severity. However, because irradiation and other forms of pretransplant conditioning have pleiotropic effects, the precise role of intestinal barrier loss in GVHD pathogenesis remains unclear. We developed GVHD models that allowed us to isolate the specific contributions of distinct pretransplant variables. Intestinal damage was required for the induction of minor mismatch [major histocompatibility complex (MHC)-matched] GVHD, but was not necessary for major mismatch GVHD, demonstrating fundamental pathogenic distinctions between these forms of disease. Moreover, recipient natural killer (NK) cells prevented minor mismatch GVHD by limiting expansion and target organ infiltration of alloreactive T cells via a perforin-dependent mechanism, revealing an immunoregulatory function of MHC-matched recipient NK cells in GVHD. Minor mismatch GVHD required MyD88-mediated Toll-like receptor 4 (TLR4) signaling on donor cells, and intestinal damage could be bypassed by parenteral lipopolysaccharide (LPS) administration, indicating a critical role for the influx of bacterial components triggered by intestinal barrier loss. In all, the data demonstrate that pretransplant conditioning plays a dual role in promoting minor mismatch GVHD by both depleting recipient NK cells and inducing intestinal barrier loss.

Identification of natural killer (NK) cells in lesions of human cutaneous graft-versus-host disease: expression of a novel NK-associated surface antigen (Kp43) in mononuclear infiltrates

We performed an immunohistochemical analysis of skin biopsies from 13 allogeneic bone marrow transplant (BMT) recipients, undergoing either acute graft-versus-host-disease (aGVHD, n = 8) or chronic GVHD (cGVHD, n = 5). A panel of different monoclonal antibodies (MoAb) was employed including anti-CD2, -CD3, -CD4, -CD8, -CD11b, -CD16, -CD56, and -CD57, as well as a recently described reagent (HP-3B1) specific for a novel natural killer (NK)-associated cell-surface antigen (Kp43). Our data indicate that in aGVHD lesions the proportions of CD2+ cells often exceeded those detected with anti-CD3 MoAb. Double labeling confirmed the presence of CD2+ CD3- lymphocytes and suggested the coexpression in some cells of CD2 and CD11b. When MoAb specific for non-lineage-restricted NK-associated markers were employed, anti-CD56 and -CD57 occasionally stained variable numbers of lymphocytes (means = 14.6% of mononuclear cells in 0.05 mm2, range less than 1-48% and means = 10.3%, range 2-25%, respectively), whereas no CD16+ lymphocytes were observed. In contrast, most samples consistently displayed substantial proportions of Kp43+ cells (means = 32.8%, range 12-63%), which appeared CD3- and were mainly located at the dermoepidermal junction. On the other hand, sections from most (four of five) cGVHD lichenoid lesions analyzed displayed lower proportions of Kp43+ and CD56+ cells. Our data point out the interest of the anti-Kp43 MoAb to identify NK cells in aGVHD lesions, suggesting their pathogenetic participation.

Cyclosporine-induced autoimmunity and immune hyperreactivity

Cyclosporine (CS) is a potent immunosuppressive agent which under some circumstances paradoxically augments DTH responses, aggravates some autoimmune diseases, and induces specific forms of autoimmunity. The enhancement of DTH and other immune responses is closely related to the timing of CS administration relative to immunization. CS inhibits IL-2 production (and several other lymphokines) at a pretranscriptional level, but does not usually prevent the antigen-specific priming of T cells, such that T cells may be poised to respond as soon as CS is withdrawn. Thus, accelerated GVHD and allograft rejection may occur after withdrawal of CS. CS has been shown to aggravate and/or induce relapse in several autoimmune diseases including collagen-induced arthritis, EAE, autoimmune thyroiditis, uveitis in SDA chickens, and an autoimmune form of myocarditis in mice. CS may enhance immune responses by inactivating suppressor cells, by altering Th1/Th2 antagonism (e.g., CS promotes a protective Th1-type response in BALB/c mice infected with Leishmania major), or by promoting T cell activation through a CS-resistant IL-2-independent T cell activation/differentiation pathway. At least three forms of CS-induced autoimmunity have been described: Syngeneic or autologous GVHD which occurs in CS-treated syngeneic or autologous bone marrow transplant recipients after CS is withdrawn in rats, mice, and humans; a systemic autoimmune disease with polyarthritis and glomerulonephritis which occurs in irradiated CBA/N mice treated with CS; and organ-specific autoimmune diseases which occur in mice treated with CS during the neonatal period. The precise mechanisms by which CS induces these autoimmune diseases are not clear, however, CS affects immune tolerance at three levels. CS induces thymic medullary involution with loss of medullary Ia+ cells, and appears to at least partially block the transition from double positive (CD4+CD8+) to single positive (mature type) thymocytes. In syngeneic bone marrow chimeras, CS appears to inhibit the intrathymic deletion of clones with relatively low affinity, but not those with high affinity, to self antigens. CS appears to inhibit the action of suppressor T cells which normally maintain an innate form of resistance to autoimmunity. Finally, CS has been shown to prevent the development of T cell clonal anergy. There is redundancy in immune tolerance mechanisms, i.e., clonal deletion, clonal anergy, and suppressor cells can each maintain tolerance to similar antigens, such that it is likely that CS must cripple more than one tolerance mechanism for autoimmunity to occur.