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-ß-receptor (LTßR) 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 LTßR-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 LTßR-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.

Donor CD4 T cells contribute to cardiac allograft vasculopathy by providing help for autoantibody production.

BACKGROUND: The development of autoantibody after heart transplantation is increasingly associated with poor graft outcome, but what triggers its development and whether it has a direct causative role in graft rejection is not clear. Here, we study the development of antinuclear autoantibody in an established mouse model of heart allograft vasculopathy. METHODS AND RESULTS: Humoral vascular changes, including endothelial complement staining, were present in bm12 heart grafts, explanted 50 days after transplantation. Alloantibody was not detectable, but long-lasting autoantibody responses developed in C57BL/6 recipients from the third week after transplantation. No autoantibody was generated if donor CD4 T cells were depleted before heart graft retrieval or in recipients that lacked B-cell major histocompatibility complex class II expression, indicating that humoral autoimmunity is a consequence of donor CD4 T-cell allorecognition of the major histocompatibility complex class II complex on recipient autoreactive B cells. An effector role for autoantibody in graft rejection was confirmed by abrogation of humoral vascular rejection, and attenuation of vasculopathy, in B-cell deficient recipients and by development of vascular obliteration and accelerated rejection in recipients primed for autoantibody before transplantation. CONCLUSIONS: Passenger CD4 T cells within heart transplants can contribute to allograft vasculopathy by providing help to recipient B cells for autoantibody generation.