CXCL13 Is an Indicator of Germinal Center Activity and Alloantibody Formation Following Transplantation.

Donor-specific antibodies (DSA) are associated with decreased graft survival following renal transplantation.1,2 Despite increased recognition of alloantibodies as mediators of early and late immunologic injury, reliable biomarkers to detect early or ongoing humoral alloreactivity have not been established. The ability to detect nascent alloantibody responses may be highly beneficial in predicting DSA formation and guiding clinical management to prevent or lower the risk of subsequent injury and premature graft failure. Although acute rejection rates in kidney transplantation are excellent for low- and short-term outcomes,3 long-term outcomes remain suboptimal and are negatively affected by the development of DSA.1,4 Solid-phase HLA antibody detection systems have greatly aided in detecting alloantibodies pretransplant and preventing hyperacute and early antibody-mediated rejection due to a preformed antibody, but their diagnostic utility posttransplant is limited. The need for biomarkers that identify DSA formation early and allow for therapeutic intervention and monitoring response to treatment persists.2,5 Durable antibody responses are a product of the germinal center (GC) reaction that occurs in secondary lymphoid organs upon antigen exposure and depends on T follicular helper (Tfh) cell interactions with cognate B cells that promote immunoglobulin class switching, the formation of plasma cells, and the subsequent production of antigen-specific antibodies.6-11 Given the dependence of antibody formation on GC reactivity and the inaccessibility of lymph node tissue, it has been postulated that circulating chemokines or cells involved in the GC reaction may function as a surrogate for antibody formation.12,13 Thus, GC-related biomarkers could indicate active humoral alloreactivity and predict the development of detectable DSA. In fact, circulating Tfh (cTfh) cells have been observed to correlate with GC activity, predict DSA after transplantation in mice,14 and parallel HLA sensitization in human renal transplant recipients.11,15 Although cTfh cells are indeed a promising biomarker for GC activity and DSA formation, they are a highly heterogenous subset that are not yet entirely understood.16 Additionally, their detection and processing, as with any potential cellular biomarker, requires isolating peripheral blood mononuclear cells with subsequent staining and flow cytometry analysis. The methods required for this analysis are cumbersome, difficult to reproduce, and high cost and require a large amount of time, which limits feasibility and can prove difficult to deploy in a clinical setting.17 Therefore, serum-based GC-associated chemokines may act as a simpler, more feasible surrogate of GC reactivity. CXCL13 (chemokine [C-X-C motif] ligand 13) is one such candidate biomarker. Alternatively referenced as B cell–attracting chemokine 1 (BCA-1), CXCL13 is the ligand for CXCR5 and is produced in lymphoid tissue by follicular cells as a homing chemokine for B cells and other CXCR5+ cells, such as Tfh cells.18,19 Together, these assist with the formation of the B-cell zone and GCs in secondary lymphoid tissues. Interestingly, CXCL13 is detectable in human blood, and its plasma levels have been shown to be associated with GC reactivity, HIV infection, and autoimmune disease activity.20,21 Because GC reactions are necessary for alloantibody formation, plasma CXCL13 levels may have the potential to also function as a biomarker for DSA formation in patients posttransplantation. Although CXCL13 has been detected in the serum, urine, and tissue of transplant recipients with allograft dysfunction and rejection,22-24 CXCL13 expression and kinetics have not been examined in response to alloantigen, nor has its potential to function as a biomarker for GC activity and subsequent DSA formation in transplantation. In this study, we sought to examine the production, kinetics, and detection of CXCL13 in a murine transplant model, as well as evaluate its potential as a biomarker in human transplant recipients with DSA.