*Purpose: Mounting evidence in kidney transplantation suggests that donor-specific antibodies (DSA) significantly contribute to renal allograft loss. However, biomarkers to guide clinical management of DSA post-transplant through the detection of nascent or ongoing humoral alloimmune responses before pathologic alloantibodies develop are not available. Therefore, there is a great need for biomarkers that can identify the presence of humoral alloreactivity and predict DSA formation. Circulating T follicular helper (Tfh) cells are CD4⁺CXCR5⁺ Tfh-like cells in the blood that correlate with humoral immune responses, but little is known about this population in transplantation.

*Methods: In this study, we utilized full MHC mismatch BALB/c to B6 and minor antigen (OVA) mismatch TCR transgenic murine skin transplant models to determine the relationship between both polyclonal endogenous and antigen-specific circulating Tfh (cTfh) cells, graft-draining lymph node germinal center (GC) reactivity and DSA formation. Transplanted mice were also treated with CD28 costimulation blockade (CoB) to evaluate the ability of cTfh cells to predict DSA in the presence of immunosuppression.

*Results: Donor-reactive CD4⁺CXCR5⁺ T cells phenotypically and functionally similar to bona fide Tfh cells expanded in the blood of skin-grafted mice post-transplantation. These cTfh cells displayed increased expression of PD-1, ICOS, TCF-1, TIGIT, and GL-7, and induced B cell class-switching and antibody production in vitro. Circulating Tfh cell kinetics and phenotypic differentiation temporally correlated with GC alloreactivity, and the emergence of an activated, effector-like ICOS⁺PD-1⁺ cTfh cell population preceded the generation of DSA (Fig.1A). Antecedent cTfh cell expansion and differentiation were not observed with selective CD28 CoB-mediated inhibition of GC reactivity and DSA formation. Conversely, cTfh cell activity was detected with less potent CoB (CTLA-4Ig) and preceded breakthrough GC reactivity and DSA. Interestingly, delayed selective CD28 CoB initiated after the detection of ICOS⁺PD-1⁺ cTfh cells in the blood post-transplant reversed the ongoing humoral alloresponse (Fig.1B) and prevented the development of DSA (Fig1.C).

*Conclusions: These findings provide experimental evidence that cTfh cells could potentially serve as a clinical biomarker for nascent or ongoing humoral alloimmune responses prior to the detection of anti-donor antibodies and inform interventional therapeutic approaches to prevent DSA.

« Back to 2019 American Transplant Congress