Delayed Cytotoxic T Lymphocyte-Associated Protein 4-Immunoglobulin Treatment Reverses Ongoing Alloantibody Responses and Rescues Allografts From Acute Rejection.

Antibody-mediated rejection has emerged as the leading cause of late graft loss in kidney transplant recipients, and inhibition of donor-specific antibody production should lead to improved transplant outcomes. The fusion protein cytotoxic T lymphocyte-associated protein 4-immunoglobulin (CTLA4-Ig) blocks T cell activation and consequently inhibits T-dependent B cell antibody production, and the current paradigm is that CTLA4-Ig is effective with naïve T cells and less so with activated or memory T cells. In this study, we used a mouse model of allosensitization to investigate the efficacy of continuous CTLA4-Ig treatment, initiated 7 or 14 days after sensitization, for inhibiting ongoing allospecific B cell responses. Delayed treatment with CTLA4-Ig collapsed the allospecific germinal center B cell response and inhibited alloantibody production. Using adoptively transferred T cell receptor transgenic T cells and a novel approach to track endogenous graft-specific T cells, we demonstrate that delayed CTLA4-Ig minimally inhibited graft-specific CD4(+) and T follicular helper responses. Remarkably, delaying CTLA4-Ig until day 6 after transplantation in a fully mismatched heart transplant model inhibited alloantibody production and prevented acute rejection, whereas transferred hyperimmune sera reversed the effects of delayed CTLA4-Ig. Collectively, our studies revealed the unexpected efficacy of CTLA4-Ig for inhibiting ongoing B cell responses even when the graft-specific T cell response was robustly established.

Reversing endogenous alloreactive B cell GC responses with anti-CD154 or CTLA-4Ig.

Alloantibodies mediate acute antibody-mediated rejection as well as chronic allograft rejection in clinical transplantation. To better understand the cellular dynamics driving antibody production, we focused on the activation and differentiation of alloreactive B cells in the draining lymph nodes and spleen following sensitization to allogeneic cells or hearts. We used a modified staining approach with a single MHC Class I tetramer (K(d)) bound to two different fluorochromes to discriminate between the Class I-binding and fluorochrome-streptavidin-binding B cells with a high degree of specificity and binding efficiency. By Day 7-8 postsensitization, there was a 1.5- to 3.2-fold increase in the total numbers of K(d) -binding B cells. Within this K(d) -binding B cell population, approximately half were IgD(low) , MHC Class II(high) and CD86(+), 30-45% expressed a germinal center (Fas(+) GL7(+)) phenotype and 3-12% were IRF4(hi) plasma cells. Remarkably, blockade with anti-CD40 or CTLA-4Ig, starting on Day 7 postimmunization for 1 or 4 weeks, completely dissolved established GCs and halted further development of the alloantibody response. Thus MHC Class I tetramers can specifically track the in vivo fate of endogenous, Class I-specific B cells and was used to demonstrate the ability of delayed treatment with anti-CD154 or CTLA-4Ig to halt established allo-B cell responses.

Matchmaking the B-cell signature of tolerance to regulatory B cells.

Confirmation of clinical tolerance requires the cessation of immunosuppressive drugs, which evoke immune reactivation and allograft rejection in all but the rare individuals who successfully transition into a state of operational transplantation tolerance. Therefore, the safe conduct of trials in transplantation tolerance requires two conditions: a sensitive and reliable means to identify individuals still being maintained on immunosuppression who are most likely to exhibit tolerance after immunosuppression is withdrawn and a noninvasive means that assesses the quality or robustness of the tolerant (TOL) state. Two recent studies attempting to identify a gene signature in peripheral blood of spontaneously TOL kidney transplant recipients made the unexpected observation that TOL, but not immune-suppressed transplant recipients, exhibited enriched B cells and B-cell transcripts in their blood. In concert with the emerging appreciation of a specialized subset of regulatory B cells (Bregs) that possess immune-modulatory function, these observations raise the possibility that Bregs play a critical role in the maintenance of tolerance to renal allografts in transplant patients. This review summarizes these recent findings and speculates on the relationship of Bregs to the maintenance of transplantation tolerance.

Blimp-1; immunoglobulin secretion and the switch to plasma cells.

The transcription factor Blimp-1 governs the generation of plasma cells and immunoglobulin secretion. Recent microarray experiments indicate that Blimp-1 regulates a large set of genes that constitute a significant part of the plasma cell expression signature. The variety of differentially expressed genes indicates that Blimp-1 affects numerous aspects of plasma cell maturation, ranging from migration, adhesion, and homeostasis, to antibody secretion. In addition, Blimp-1 regulates immunoglobulin secretion by affecting the nuclear processing of the mRNA transcript and by affecting protein trafficking by regulating genes that impact on the activity of the endoplasmic reticulum. Interestingly, the differentiation events that Blimp-1 regulates appear to be modulated depending on the activation state of the B cell. This modulation may be due at least in part to distinct regions of Blimp-1 that regulate unique sets of genes independently of each other. These data hint at the complexity of Blimp-1 and the genetic program that it initiates to produce a pool of plasma cells necessary for specific immunity.

TCRgammadelta cells and viruses.

T-cell receptor gammadelta cells (TCRgammadelta) are often found in increased numbers during the course of several viral infections in humans. Although these findings suggest an important role for this unique subset, their precise function has not been ascertained. Recent studies in murine models of both RNA and DNA virus infections have begun to shed new light on the potential function for TCRgammadelta cells in antiviral immunity. It is clear that TCRgammadelta cells participate in the immune response to human immunodeficiency virus (HIV), influenza, Sendai, coxsackie, vaccinia, vesicular stomatitis virus (VSV), and herpes simplex virus-1 (HSV-1) viral infections since they become activated and home to the sites of viral replication. In this review we will summarize current efforts to dissect the role of TCRgammadelta cells in these disease settings, emphasizing the effector functions utilized, the TCR repertoire, and the antigens recognized. Particular focus will be placed on HSV-1 infections where we have begun to address these issues and have shown that TCRgammadelta cells are sufficient for protection from lethal infection and are able to recognize the herpes virus antigen glycoprotein I.

HSV-1 glycoprotein I-reactive TCR gamma delta cells directly recognize the peptide backbone in a conformationally dependent manner.

Despite the description of numerous antigenic ligands recognized by TCRgammadelta cells, detailed information concerning the structural nature of these antigenic epitopes is lacking. In addition, the recent descriptions of human TCRgammadelta cells recognizing mycobacterium-derived low m.w. lipid molecules confirms that the spectrum and nature of biologic structures that are capable of being recognized by TCRgammadelta cells are unclear. We have previously described a murine TCRgammadelta cell clone, TgI4.4, that is reactive to herpes simplex virus (HSV)-1 glycoprotein I (gI). Unlike TCRalphabeta-mediated, MHC-restricted Ag recognition but similar to Ig Ag recognition, TgI4.4 recognizes purified gI directly, in the absence of Ag processing or presentation. Since gI is a complex glycoprotein, the nature of the antigenic epitope was investigated. First, gI recognition by TgI4.4 is conformationally dependent, as revealed by denaturation and proteolytic experiments. Secondly, the epitope recognized by TgI4.4 was mapped to the amino terminus by using insertion mutants of gI. Lastly, TgI4.4 recognizes the gI protein directly since completely deglycosylated forms of gI are efficiently recognized. Therefore, TCRgammadelta cells are capable of recognizing a variety of molecular structures, including proteins. The ability of TgI4.4 to recognize a nonglycosylated form of gI suggests that HSV-1 recognition by TCRgammadelta cells in vivo is not limited by cell-specific glycosylation patterns or glycosylation-dependent conformational influences.

T cell receptor-gamma/delta cells protect mice from herpes simplex virus type 1-induced lethal encephalitis.

Increased numbers of T cell receptor (TCR)-gamma/delta cells have been observed in animal models of influenza and sendai virus infections, as well as in patients infected with human immunodeficiency virus and herpes simplex virus type 1 (HSV-1). However, a direct role for TCR-gamma/delta cells in protective immunity for pathogenic viral infection has not been demonstrated. To define the role of TCR-gamma/delta cells in anti-HSV-1 immunity, TCR-alpha-/- mice treated with anti- TCR-gamma/delta monoclonal antibodies or TCR-gamma/delta x TCR-alpha/beta double-deficient mice were infected with HSV-1 by footpad or ocular routes of infection. In both models of HSV-1 infection, TCR-gamma/delta cells limited severe HSV-1-induced epithelial lesions and greatly reduced mortality by preventing the development of lethal viral encephalitis. The observed protection resulted from TCR-gamma/delta cell-mediated arrest of both viral replication and neurovirulence. The demonstration that TCR-gamma/delta cells play an important protective role in murine HSV-1 infections supports their potential contribution to the immune responses in human HSV-1 infection. Thus, this study demonstrates that TCR-gamma/delta cells may play an important regulatory role in human HSV-1 infections.

TCR gamma delta cells: a specialized T-cell subset in the immune system.

Specificity, memory, and self/nonself discrimination are the essential principles that underlie the acquired immune response. From birth through one's life, the immune system is continually responding to new environmental challenges (e.g. bacteria and viruses) and developing a specific, long-lasting immunity to those challenges. The first exposure to a pathogen results in the recruitment of multiple cell types, including macrophages, dendritic cells, and assorted leukocytes, that initiate an antigen nonspecific inflammatory response designed to attract T cells and B cells to the inflammatory sites including the draining lymph nodes. The foreign antigens are concentrated within the professional antigen-presenting cells (APCs) that process and present small antigenic peptides to CD4+ and CD8+ T cells in association with class II and class I major histocompatibility complex (MHC) molecules, respectively. The activated CD4+ T-cell receptor (TCR) alpha beta cells respond vigorously to the pathogen in an antigen-specific manner, liberating a barrage of cytokines that induce B cells to differentiate to antibody-secreting plasma cells and likewise cause CD8+ cells to differentiate into cytolytic effectors. The T cells and B cells expand in an evolving, highly specific manner to control the initial infection while developing long-term acquired immunity such that any further infection by that pathogen is virtually impossible. Thus TCR alpha beta T cells are the central lymphocyte in the immune system, providing specific pathogen recognition and long-term memory all within the context of distinguishing foreign from self antigens. Yet, during a primary immune response, there is a lag time of approximately 3 to 4 days before antigen-specific responses are evident, and TCR alpha beta responses do not peak until approximately day 7. Therefore, it is essential that other strategies are employed by the immune system in order to mount an aggressive early immune response.

Unique antigen recognition by a herpesvirus-specific TCR-gamma delta cell.

TCR-gamma delta cells, a T cell subset present in the epithelial and lymphoid tissues, have been implicated in viral and bacterial infections. We have identified a TCR-gamma delta clone (TgI4.4) that, unlike TCR-alpha beta cells, recognizes a herpes simplex virus type 1 transmembrane glycoprotein, gI, in an MHC class I- and class II-independent fashion. The TCR of TgI4.4 is composed of rearranged V delta 8 (a V alpha 2 family member) and V gamma 1.2 variable genes, a heterodimeric pair not previously described. Furthermore, anti-V alpha 2 mAbs are sufficient to block recognition of the gI ligand. Strikingly, anti-gI Abs also are capable of blocking recognition, a phenomena that is very rare in TCR-alpha beta Ag recognition. Therefore, to dissect the mechanism involved in this unique form of Ag recognition, we constructed a mutant of gI, gIt, that lacks cell surface expression upon transfection into APCs. This form of gI was not sufficient for Ag presentation. In contrast, wild-type gI expressed in the Ag-processing mutant cell, RMA-S, is recognized by TgI4.4, suggesting that gI presentation occurs independently of classical Ag-processing pathways. In fact, through the use of a soluble recombinant gI molecule, gI-Ig, we show that TgI4.4 can recognize whole, unprocessed gI protein in the absence of any APCs. These results suggest that there exist alternate and novel forms of TCR Ag recognition, and that the TCR-gamma delta clone, TgI4.4, may represent a novel T cell subset that, during pathogenic challenge, may respond directly to Ags on the surfaces of bacteria and viruses.

TCR gamma delta cells: mysterious cells of the immune system.

Gamma delta cells participate in pathogenic infections and autoimmune conditions, yet, almost a decade after their discovery, little is known regarding their TCR repertoire or effector functions. Unlike MHC-restricted antigen recognition employed by TCR alpha beta cells, TCR gamma delta cells can recognize whole unprocessed antigens in an MHC- independent manner. The nature of positive and negative selection used to shape the repertoire of TCR gamma delta cells is unclear, especially in the nonlymphoid tissues where these cells predominate. While TCR gamma delta cells express an activated phenotype and are present in pathological conditions, their roles in immunological protein is unknown. This review will focus on our efforts to study these issues of TCR gamma delta biology.