T Cells in Transplantation and Tissue Injury
Our laboratory focuses on immune mechanisms in transplantation and tissue injury with a particular focus on T cells and the regulatory circuits involved in these responses.
T Cell Subsets Involved in Transplant Rejection
Organ transplantation is a common life-saving modality but problems that affect long-term outcomes remain some of the most challenging and complex areas of medicine. Over the last several years, we have witnessed only moderate improvements in transplant outcomes and have not been able to prevent chronic rejection, currently the major factor limiting long-term survival of transplants. The development of new methods to improve transplant outcome has been hindered by the lack of a comprehensive understanding of the cell types involved in the rejection process and mechanisms underlying rejection. Th1 cells have classically been considered to be the main effector cell type in rejection. However, recent data suggest that Th17 cells also play a role in the rejection process, albeit poorly understood. We have shown that in the absence of a Th1 response, IL-17 producing CD4 and CD8 T cells (T17 cells) are able to mediate allograft rejection. Our lab is attempting to define the role of Th17 cells in rejection responses and to examine the plasticity of Th17 cells during rejection and how these cells may be controlled either by regulatory mechanisms or costimulation.
Figure 1. IL-17 producing CD4 and CD8 T cells infiltrate rejecting cardiac transplants. The panels on the right are overlays showing IL-17 production by CD4 or CD8 T cells.
Regulation of Tissue Injury Responses by microRNAs
Ischemic injury followed by reperfusion results in the clinical syndrome of acute kidney injury, a common clinical problem. The initial non-immune hypoxic injury and subsequent reperfusion leads to activation of innate and adaptive immune responses, resulting in tissue damage. Following injury, a repair process involving cellular proliferation must take place in order to regain renal function. IRI is also inevitable in kidney transplantation and contributes to delayed graft function and long-term changes in kidney transplants that affect outcome. While IRI is clearly a major clinical problem in native kidneys and in the setting of renal transplantation, the pathogenesis of renal IRI is not fully understood. My laboratory has begun to examine how microRNAs, a class of small, noncoding RNAs approximately 21-22 nucleotides in length that regulate gene expression post-transcriptionally, may regulate injury and repair process in the kidney using IRI as an injury model. microRNA expression profiling of RNA from kidneys subject to IRI suggest that differential expression of microRNAs may serve as a biomarker of renal injury. Moreover, we have begun to define the physiological role of differentially expressed microRNAs in injury and repair resulting from IRI. More recently we have begun to extend this work into kidney injury resulting from transplantation related processes such as cold ischemia and rejection.
Figure 2. Principal component analysis reveals that microRNA expression profiles may serve as a biomarker of renal injury. Shown is a 3-D plot of the first three principal components of microRNA expression data for C57BL/6 mice undergoing either a sham procedure (red line), or IRI (blue line), Rag-1-/- mice undergoing either a sham procedure (solid grey line) or IRI (dotted grey line), or Rag-2/cγ-/- mice undergoing either a sham procedure (solid black line) or IRI (dotted black line). Samples for naïve C57BL/6 mice are show as a black dot. Samples for naïve Rag-1-/- and or Rag-2/cγ-/- mice are shown as grey and black stars, respectively. Shapiro et al, submitted.
microRNAs in T Cell Activation
T cell activation is a highly regulated process that requires the coordination of several sequential events that function to drive T cells to a differentiated state. Activation requires signaling through the T cell receptor (TCR) upon recognition of peptide-MHC complexes on the surface of antigen presenting cells, and the delivery of co-stimulatory signals. The CD28 co-stimulatory pathway plays a central role in activating signaling pathways, such as the PI3K pathway, that promote T cell survival, cytokine production and differentiation. Given the emerging role of microRNAs in the control of lymphocyte development and function we set out to examine potential roles of microRNAs in T cell activation. Through genome-wide miRNA expression profiling, we identified a relatively limited number of microRNAs that were either up or down-regulated after T cell activation. Our data show that miR-214 targets Pten in activated T cells and that its expression leads to increased T cell proliferation. Furthermore, up-regulation of miR-214 was dependent on CD28 costimulation. Our results therefore suggest that costimulation through CD28 overcomes negative regulation of TCR signaling by allowing for increased expression of miR-214 which in turn regulates Pten expression. Thus, the requirement for T cell costimulation is in part related to its ability to regulate expression of microRNAs that control T cell activation. We are now attempting to understand how differential expression of microRNAs upon T cell activation is coordinated, define the physiological role of other microRNAs discovered to be differentially regulated and understand how microRNAs may work in concert to regulate T cell responses.
Natural antibodies that bind the carbohydrate antigen Galα1-3Galβ1-4GlcNAc-R (Gal) comprise a significant proportion of natural antibodies in humans, apes and Old World primates. Gal-reactive natural antibodies are estimated to comprise between one and eight percent of circulating immunoglobulin in humans, and it has been shown that approximately one percent of Epstein-Barr virus-transformed peripheral blood B cells make antibodies that bind Gal. All other placental mammals express Gal epitopes on most tissues, and are consequently tolerant to this antigen. The natural antibody repertoire of humans, apes and Old World primates is therefore distinct from that of all other mammalian species. Although the role of natural antibodies in humans has not been tested experimentally, it has been proposed that natural antibodies that bind Gal play an important role in protective host immunity.
Natural antibodies have also been suggested to be involved in concentrating antigen to lymphoid organs, which can enhance antigen-specific T- and B-cell responses by forming immune complexes. It has been shown that administration of antibody-antigen immune complexes to mice induces antibody responses that are several hundred-fold higher than administration of antigen alone. The ability of natural antibodies to fix complement may also play a role in enhancing immune responses by targeting antigen to dendritic cells that express complement receptors, as well as activation of B cells through CD21 dependent mechanisms. Expression of Gal-specific antibodies on the surface of B cells could also affect immune responses by providing a means to efficiently capture Gal-modified antigens that can then be processed and presented to T cells.
Data for our lab suggests that the presence of antibodies specific for Gal encoded in the natural antibody repertoire increase B and T cell responses to poorly immunogenic antigens that have been modified to express Gal epitopes. We are building upon these observations to understand whether pre-existing natural antibody repertoires can be used to augment vaccines. We are also using immunoglobulin knock-in mice we constructed to understand how B cells producing natural antibodies are regulated developmentally and why this antigen specificity is restricted to marginal zone B cells.
Figure 3. A model of how natural antibodies encoded for by marginal zone B cells may augment immunogenicity.