Reza Motallebzadeh

Germinal Center Alloantibody Responses Are Mediated Exclusively by Indirect-Pathway CD4 T Follicular Helper Cells

The durable alloantibody responses that develop in organ transplant patients indicate long-lived plasma cell output from T-dependent germinal centers (GCs), but which of the two pathways of CD4 T cell allorecognition is responsible for generating allospecific T follicular helper cells remains unclear. This was addressed by reconstituting T cell-deficient mice with monoclonal populations of TCR-transgenic CD4 T cells that recognized alloantigen only as conformationally intact protein (direct pathway) or only as self-restricted allopeptide (indirect pathway) and then assessing the alloantibody response to a heart graft. Recipients reconstituted with indirect-pathway CD4 T cells developed long-lasting IgG alloantibody responses, with splenic GCs and allospecific bone marrow plasma cells readily detectable 50 d after heart transplantation. Differentiation of the transferred CD4 T cells into T follicular helper cells was confirmed by follicular localization and by acquisition of signature phenotype. In contrast, IgG alloantibody was not detectable in recipient mice reconstituted with direct-pathway CD4 T cells. Neither prolongation of the response by preventing NK cell killing of donor dendritic cells nor prior immunization to develop CD4 T cell memory altered the inability of the direct pathway to provide allospecific B cell help. CD4 T cell help for GC alloantibody responses is provided exclusively via the indirect-allorecognition pathway.

Blocking lymphotoxin signaling abrogates the development of ectopic lymphoid tissue within cardiac allografts and inhibits effector antibody responses

Tertiary lymphoid organs (TLOs) may develop within allografts, but their contribution to graft rejection remains unclear. Here, we study a mouse model of autoantibody‐mediated cardiac allograft vasculopathy to clarify the alloimmune responses mediated by intragraft TLOs and whether blocking lymphotoxin‐β‐receptor (LTβR) signaling, a pathway essential for lymphoid organogenesis, abrogates TLO development. TLOs (defined as discrete lymphoid aggregates associated with high endothelial venules) were detectable in 9 of 13 heart allografts studied and were predominantly B cell in composition, harboring germinal‐center activity. These are most likely manifestations of the humoral autoimmunity triggered in this model after transplantation; TLOs did not develop if autoantibody production was prevented. Treatment with inhibitory LTβR‐Ig fusion protein virtually abolished allograft TLO formation (mean TLOs/heart: 0.2 vs. 2.2 in control recipients; P=0.02), with marked attenuation of the autoantibody response. Recipients primed for autoantibody before transplantation rejected grafts rapidly, but this accelerated rejection was prevented by postoperative administration of LTβR‐Ig (median survival time: 18 vs. >50 d, respectively, P=0.003). Our results provide the first demonstration that TLOs develop within chronically rejecting heart allografts, are predominantly B cell in origin, and can be targeted pharmacologically to inhibit effector humoral responses.—Motallebzadeh, R., Rehakova, S., Conlon, T. M., Win, T. S., Callaghan, C. J., Goddard, M., Bolton, E. M., Ruddle, N. H., Bradley, J. A., Pettigrew, G. J. Blocking lymphotoxin signaling abrogates the development of ectopic lymphoid tissue within cardiac allografts and inhibits effector antibody responses. FASEB J. 26, 51–62 (2012). www.fasebj.org

Copresentation of Intact and Processed MHC Alloantigen by Recipient Dendritic Cells Enables Delivery of Linked Help to Alloreactive CD8 T Cells by Indirect-Pathway CD4 T Cells

In transplantation, direct-pathway CD8 T cells that recognize alloantigen on donor cells require CD4 help for activation and cytolytic function. The ability of indirect-pathway CD4 T cells to provide this help remains unexplained, because a fundamental requirement for epitope linkage is seemingly broken. The simultaneous presentation, by host dendritic cells (DCs), of both intact MHC class I alloantigen and processed alloantigen would deliver linked help, but has not been demonstrated definitively. In this study, we report that following in vitro coculture with BALB/c DCs, small numbers (∼1.5%) of C57BL/6 (B6) DCs presented acquired H-2d alloantigen both as processed allopeptide and as unprocessed Ag. This represented class I alloantigen provides a conformational epitope for direct-pathway allorecognition, because B6 DCs isolated from cocultures and transferred to naive B6 mice provoked cytotoxic CD8 T cell alloimmunity. Crucially, this response was dependent upon simultaneous presentation of class II–restricted allopeptide, because despite acquiring similar amounts of H-2d alloantigen upon coculture, MHC class II–deficient B6 DCs failed to elicit cytotoxic alloimmunity. The relevance of this pathway to solid-organ transplantation was then confirmed by the demonstration that CD8 T cell cytotoxicity was provoked in secondary recipients by transfer of DCs purified from wild-type, but not from MHC class II–deficient, C57BL/6 recipients of BALB/c heart transplants. These experiments demonstrate that representation of conformationally intact MHC alloantigen by recipient APC can induce cytotoxic alloimmunity, but simultaneous copresentation of processed allopeptide is essential, presumably because this facilitates linked recognition by indirect-pathway CD4 Th cells.

Lymphoid Tissue Formation in Allografts: Innocent Until Proven Guilty

Tertiary lymphoid organs (TLOs) are lymphoid-like structures commonly found at sites of chronic inflammation. Commonly associated with autoimmune responses, they have recently been described within organ allografts. Although TLOs are similar structurally to secondary lymphoid organs, their function remains largely unknown. In this review, we discuss how immune responses within TLOs may influence graft survival.

Regulation of Allograft Survival by Inhibitory FcγRIIb Signaling

Fcγ receptors (FcγR) provide important immunoregulation. Targeting inhibitory FcγRIIb may therefore prolong allograft survival, but its role in transplantation has not been addressed. FcγRIIb signaling was examined in murine models of acute or chronic cardiac allograft rejection by transplanting recipients that either lacked FcγRIIb expression (FcγRIIb−/−) or overexpressed FcγRIIb on B cells (B cell transgenic [BTG]). Acute heart allograft rejection occurred at the same tempo in FcγRIIb−/− C57BL/6 (B6) recipients as wild type recipients, with similar IgG alloantibody responses. In contrast, chronic rejection of MHC class II–mismatched bm12 cardiac allografts was accelerated in FcγRIIb−/− mice, with development of more severe transplant arteriopathy and markedly augmented effector autoantibody production. Autoantibody production was inhibited and rejection was delayed in BTG recipients. Similarly, whereas MHC class I–mismatched B6.Kd hearts survived indefinitely and remained disease free in B6 mice, much stronger alloantibody responses and progressive graft arteriopathy developed in FcγRIIb−/− recipients. Notably, FcγRIIb-mediated inhibition of B6.Kd heart graft rejection was abrogated by increasing T cell help through transfer of additional H2.Kd-specific CD4 T cells. Thus, inhibitory FcγRIIb signaling regulates chronic but not acute rejection, most likely because the supra-optimal helper CD4 T cell response in acute rejection overcomes FcγRIIb-mediated inhibition of the effector B cell population. Immunomodulation of FcγRIIb in clinical transplantation may hold potential for inhibiting progression of transplant arteriopathy and prolonging transplant survival.

Augmentation of Recipient Adaptive Alloimmunity by Donor Passenger Lymphocytes within the Transplant

Summary Chronic rejection of solid organ allografts remains the major cause of transplant failure. Donor-derived tissue-resident lymphocytes are transferred to the recipient during transplantation, but their impact on alloimmunity is unknown. Using mouse cardiac transplant models, we show that graft-versus-host recognition by passenger donor CD4 T cells markedly augments recipient cellular and humoral alloimmunity, resulting in more severe allograft vasculopathy and early graft failure. This augmentation is enhanced when donors were pre-sensitized to the recipient, is dependent upon avoidance of host NK cell recognition, and is partly due to provision of cognate help for allo-specific B cells from donor CD4 T cells recognizing B cell MHC class II in a peptide-degenerate manner. Passenger donor lymphocytes may therefore influence recipient alloimmune responses and represent a therapeutic target in solid organ transplantation.

Diversity of the CD4 T cell alloresponse: the short and the long of it

Summary MHC alloantigen is recognized by two pathways: “directly,” intact on donor cells, or “indirectly,” as self-restricted allopeptide. The duration of each pathway, and its relative contribution to allograft vasculopathy, remain unclear. Using a murine model of chronic allograft rejection, we report that direct-pathway CD4 T cell alloresponses, as well as indirect-pathway responses against MHC class II alloantigen, are curtailed by rapid elimination of donor hematopoietic antigen-presenting cells. In contrast, persistent presentation of epitope resulted in continual division and less-profound contraction of the class I allopeptide-specific CD4 T cell population, with approximately 10,000-fold more cells persisting than following acute allograft rejection. This expanded population nevertheless displayed sub-optimal anamnestic responses and was unable to provide co-stimulation-independent help for generating alloantibody. Indirect-pathway CD4 T cell responses are heterogeneous. Appreciation that responses against particular alloantigens dominate at late time points will likely inform development of strategies aimed at improving transplant outcomes.

Unlinked Memory Helper Responses Promote Long-Lasting Humoral Alloimmunity

Essential help for long-lived alloantibody responses is theoretically provided only by CD4 T cells that recognize target alloantigen, processed and presented by the allospecific B cell. We demonstrate that in an alloresponse to multiple MHC disparities, cognate help for class-switched alloantibody may also be provided by CD4 T cells specific for a second “helper” alloantigen. This response was much shorter-lived than when help was provided conventionally, by Th cell recognition of target alloantigen. Nevertheless, long-lasting humoral alloimmunity developed when T cell memory against the helper alloantigen was first generated. Costimulatory blockade abrogated alloantibody produced through naive Th cell recognition of target alloantigen but, crucially, blockade was ineffective when help was provided by memory responses to the accessory helper alloantigen. These results suggest that memory Th cell responses against previously encountered graft alloantigen may be the dominant mechanism for providing help to generate new specificities of alloantibody in transplant patients receiving immunosuppression.

High alcohol intake in deceased donors has no effect on pancreas graft survival: a registry analysis

Outcomes of pancreas transplantation from donors with high alcohol consumption are poorly described. The UK Transplant Registry was used to determine whether donor alcohol intake influenced pancreas survival in simultaneous pancreas–kidney (SPK) transplants performed between 2006 and 2012 (n = 770). Recipients were stratified by donor alcohol intake: group I (n = 122)—high recent alcohol intake (>21 or >14 units of alcohol/week in males or females, respectively) or previous alcohol abuse and group II (n = 648)—low/unknown current intake and no previous alcohol abuse. Median current alcohol intake was higher in group I than group II: 36.3 vs. 10 units/week; P < 0.001. One‐ and five‐year pancreas graft survivals were 88.5% and 73.6% in group I, and 87% and 74.9% in group II. There was no difference in unadjusted graft survival between groups I and II (P = 0.76), and no difference between group II and a subgroup of group I with a donor history of alcohol abuse and high current intake (P = 0.26), or from donors with current alcohol consumption of >50 units/week (P = 0.41). Pancreas donors with past alcohol abuse or current high intake are common, and graft outcomes appear to be acceptable. This analysis suggests that high donor alcohol intake, by itself, should not exclude consideration of pancreas transplantation.

Are donor lymphocytes a barrier to transplantation tolerance

Purpose of review Following solid organ transplantation (SOT), populations of donor lymphocytes are frequently found in the recipient circulation. Their impact on host alloimmunity has long been debated but remains unclear, and it has been suggested that transferred donor lymphocytes may either promote tolerance to the graft or hasten its rejection. We discuss possible mechanisms by which the interaction of donor passenger lymphocytes with recipient immune cells may either augment the host alloimmune response or inhibit it. Recent findings Recent work has highlighted that donor T lymphocytes are the most numerous of the donor leukocyte populations within a SOT and that these may be transferred to the recipient after transplantation. Surprisingly, graft-versus-host recognition of major histocompatibility complex class II on host B cells by transferred donor CD4 T cells can result in marked augmentation of host humoral alloimmunity and lead to early graft failure. Killing of donor CD4 T cells by host natural killer cells is critical in preventing this augmentation. Summary The ability of passenger donor CD4 T cells to effect long-term augmentation of the host humoral alloimmune response raises the possibility that ex-vivo treatment or modification of the donor organ prior to implantation may improve long-term transplant outcomes.