Immune Mechanisms in Cardiac Graft Rejection Explored: Future Therapies In Sight

Often, when an individual has a severely damaged tissue or organ, organ transplantation by a donor may be the only option to ensure the survival of this individual. As organ transplantation procedures have become more successful in recent years, further research is being conducted to identify therapies with improved success rates of these operations. A problem often associated with allotransplantation, or the transfer of tissues or organs from one individual to another of the same species, is graft rejection mediated by the host’s immune system. Acute graft rejection often takes place in the days following a transplant, and can result in transplanted tissue damage, development of chronic graft rejection, or in the worst-case scenario, transplanted organ failure.

Several processes have been identified in the host rejection of transplanted tissue. After transplantation of a donor organ, recipient dendritic cells (DCs) encounter “foreign” antigen from the donor tissue that appear to be dangerous to the host, and present small bits of its proteins, or peptides, to host B cells. These B cells then produce specific antibodies (Abs) that initiate other processes leading to the destruction of the foreign graft tissue. More often in graft rejection, however, the donor’s DCs present the donor’s own “self” peptides to host helper T cells, also called CD4 T cells, that have not yet encountered these foreign peptides. These CD4 T cells then may either help to establish cytotoxic (CD8) T cells that mount a T_H1-dominated response in attempt to destroy the dangerous cells, characterized by production of inflammatory cytokines, or the CD4 T cells may themselves mount a T_H2 immune response, typified by production of eosinophils, cells that secrete toxic granules that help counter foreign or potentially dangerous cells. Obviously, however, when a patient receives a transplanted tissue or organ, the survival of the transplanted tissue is critical for the success of the transplant, and thus these immune processes must be dampened to ensure that this occurs.

Typically, patients who undergo organ or tissue allotransplants are treated with immunosuppressants to prevent acute graft rejection. Corticosteroids are used most often to dampen the immune response and to counter the inflammation and consequent tissue damage that may occur, but they possess several severe side effects in addition to immunosuppression, and may lead to the recipient developing other infectious diseases as a result. Current therapeutic research is thus targeted at elucidating the molecular processes involved in graft rejection, which may ultimately yield treatments that can target specific molecular pathways without inducing such broad immunosuppressive effects. Just earlier this month, a study was published in The Journal of Immunology by Booth and colleagues that identifies a potential mechanism by which the host immune system induces cardiac graft rejection. The paper implicates a critical role for the cytokine IL-6 in graft infiltration by host lymphocytes (T cells and B cells) and the shaping of CD4 T cell response associated with CD4-dominant cardiac graft rejection. To achieve this, the authors utilized a mouse model in which they performed cardiac transplant and examined the role of IL-6 in either CD4- or CD8-dominated responses.

Booth and colleagues chose to examine IL-6, specifically, because its role in promoting acute graft rejection is poorly understood, and the authors had recently described a critical role for IL-6 in chronic allograft rejection (1). To begin, the authors sought to examine how IL-6 alters the alloimmune response, or in other words, the immune response against the transplanted cardiac tissue. It was already known that in mice containing both CD4 and CD8 cells, cardiac graft rejection is dominated by CD8 cells that mount a T_H1 response (2). Recipient mice were treated with a monoclonal antibody (mAb) that neutralizes IL-6 (in other words, rendering it dysfunctional), fittingly called anti-IL-6, at the day prior to transplant, one day post-transplant, and three days post transplant. What the authors observed was that the allografts survived significantly longer in these mice than in mice treated with a control Ab. Intriguingly, the authors also observed in the mice lacking IL-6 a reduction in graft-infiltrating cells (GICs). Further characterization of those GICs present indicated a higher percentage of eosinophils, suggesting an increased T_H2 response. From this, the authors concluded that IL-6 must promote a T_H1 response dominated by CD8 cells, and that in the absence of IL-6, CD8 cell-dominated graft rejection is delayed.

Subsequently, Booth et al. sought to determine if IL-6 was necessary for acute rejection of cardiac grafts mediated by CD4 T cells. To do this, they treated allograft recipient mice with a mAb that depleted their CD8 cell populations, and then treated those mice with either the anti-IL-6 mAb or a control Ab. Mice treated with anti-IL-6 showed considerably longer graft survival, many for at least 50 days post transplant, and a sizable decrease in GICs. This was accompanied by only a small decrease in T_H1 cytokine production, suggesting that the CD4-mediated graft rejection primarily prevented graft infiltration. With this information, the authors sought to clarify these roles of IL-6. This was accomplished by delaying injection of anti-IL-6 in CD8-depleted recipient mice by up to 6 days after the transplant procedure. In this case, delayed neutralization of IL-6 still resulted in prolonged graft survival and reduced T_H1 response similar to mice whose IL-6 was neutralized prior to transplant.

Based on this information, the authors hypothesized that IL-6 may play a role in promoting the migration of host lymphocytes to the graft. IL-6 has previously been implicated to promote production of MCP-1, a chemokine protein that directs lymphocyte migration along a chemokine gradient (3). Therefore, the authors sought to determine if IL-6 neutralization might have an effect on MCP-1 expression, and found that expression of MCP-1 in the graft site was indeed significantly reduced when IL-6 was neutralized, suggesting a likely explanation for the reduction in GICs. Additionally, since mice whose CD8 cells were depleted were skewed towards a T_H2 response, and these T_H2 responses are associated with Abs that may play a role in cardiac rejection, the authors examined host Ab production in anti-IL-6 mice. Results indicated a decrease in graft-reactive IgG, a specific type of Ab, in mice with neutralized IL-6, suggesting that IL-6 may further contribute to graft rejection by upregulating production of anti-graft Ab.

Lastly, Booth and colleagues expected IL-6 neutralization to affect both graft and recipient supplies of IL-6. They tested this hypothesis by examining graft rejection in IL-6 -/- (IL-6 deficient) recipients in CD4 dominated rejection, and found that grafts in IL-6 deficient mice survived significantly longer—comparable to grafts that received anti-IL-6 mAb. Like the anti-IL-6 mice, these IL-6 deficient mice also possessed grafts with reduced GICs and decreased T_H1 response.

All in all, this study indicated that in cardiac transplantation, recipient Il-6 production is critical for graft infiltration and CD4-mediated recipient immune responses in mice. As these processes are analogous to those in humans, this provides significant insight into the mechanisms underlying cardiac graft rejection and directs potential future therapeutic strategies for dampening the immune response after tissue transplant, while not inducing the broad, non-focused immunosuppressive side effects of current treatments. With information from this study, future research for possible novel treatments to enhance current immunosuppressive drugs should target the IL-6 signaling pathways, thereby decreasing incidence of both acute and chronic cardiac allograft rejection. Once developed, these drugs could to go to clinical trials, bringing transplant recipients that much closer to receiving better, more focused immunosuppressive treatment with fewer side effects.

Reference: Booth, A. J., Grabauskiene, S., Wood, S. C., Lu, G., Burrell, B. E., & Bishop, D. K. (2011). IL-6 Promotes Cardiac Graft Rejection Mediated by CD4+ Cells. Journal of immunology (Baltimore, Md. : 1950), 187(11), 5764-71. doi:10.4049/jimmunol.1100766

Other References:

(1) Booth, a J., Csencsits-Smith, K., Wood, S. C., Lu, G., Lipson, K. E., & Bishop, D. K. (2010). Connective tissue growth factor promotes fibrosis downstream of TGFbeta and IL-6 in chronic cardiac allograft rejection. American journal of transplantation : official journal of the American Society of Transplantation and the American Society of Transplant Surgeons, 10(2), 220-30. doi:10.1111/j.1600-6143.2009.02826.x

(2) Chan, S. Y., L. A. DeBruyne, R. E. Goodman, E. J. Eichwald, and D. K. Bishop. 1995. In vivo depletion of CD8+ T cells results in Th2 cytokine production and alternate mechanisms of allograft rejection. Transplantation 59: 1155–1161.

(3) Spörri, B., Müller, K. M., Wiesmann, U., & Bickel, M. (1999). Soluble IL-6 receptor induces calcium flux and selectively modulates chemokine expression in human dermal fibroblasts. International immunology, 11(7), 1053-8. Retrieved from http://www.ncbi.nlm.nih.gov/pubmed/10383937

Image: http://beverlyhillscardiology.com/heart-transplantation-2/