Selective CD28 Blockade Exhibits Superior Suppression of Donor-Specific Antibody Relative to CTLA4Ig

Costimulation blockade in the form of CTLA4Ig is known to attenuate donor-specific antibody responses. However, CTLA4Ig indiscriminately blocks both the CD28 costimulatory and CTLA4 co-inhibitory pathways. While follicular helper T (TFH) cell development and germinal center (GC) formation are CD28-dependent, CTLA4 has been shown to play a critical role in suppressing the provision of T cell help for GC B cell function and immunoglobulin production. Thus, selective CD28 blockade has potential to inhibit GC development while garnering the inhibitory effects of intact CTLA4 signaling on T cell-dependent antibody responses. To test this possibility, we used a T cell receptor transgenic murine skin allograft model to compare a novel CD28-specific domain antibody (dAb) to CTLA4Ig. The selective CD28 dAb resulted in superior skin allograft survival in comparison to CTLA4Ig (MST >100d vs. 34d). Importantly, animals treated with the anti-CD28 dAb experienced a greater reduction in donor-specific IgG as compared to those treated with CTLA4Ig. Flow cytometric analysis of donor-reactive CD4+Thy1.1+ T cells from draining lymph nodes revealed increased expression of CTLA4 on TFH cells (CXCR5+GL7+/-) compared to naïve and non-TFH cells (CXCR5-GL7-), with the greatest level of CTLA4 expression on maximally polarized GC TFH cells (CXCR5+GL7+). Analysis of donor-specific CD4+Thy1.1+ T cells also revealed a greater reduction in the frequency and absolute number of TFH cells (CXCR5+GL7+/-) with selective CD28 blockade as compared to CTLA4Ig. Additionally, we observed a reciprocal increase in the frequency and absolute number of naïve and non-TFH cells (CXCR5-GL7-) that was greatest with CD28 dAb treatment. These findings suggest that selective CD28 blockade in the presence of intact CTLA4 signaling has a greater inhibitory effect on T cell dependent antibody responses than CTLA4Ig. DISCLOSURE:Suchard, S.: Employee, Bristol-Myers Squibb. Nadler, S.: Employee, Bristol-Myers Squibb.