Pretransplant Desensitization with Costimulation Blockade and Proteasome Inhibitor Reduces DSA and Delays Antibody-Mediated Rejection in Highly Sensitized Nonhuman Primate Kidney Transplant Recipients
Authors: Ezekian Brian , Schroder Paul M. , Mulvihill Michael S. , Barbas Andrew , Collins Bradley , Freischlag Kyle , Yoon Janghoon , Yi John S. , Smith Felicitas , Olaso Danae , Saccoccio Frances M. , Permar Sallie , Farris Alton B. , Kwun Jean , Knechtle Stuart J. 
Published: October 2019

Significance Statement
Although common desensitization strategies for patients with broad HLA sensitization permit transplantation via transient lowering of donor-specific antibodies, the B cell–response axis from germinal center activation to plasma cell differentiation remains intact. In this study, dual-targeting desensitization with carfilzomib (a proteasome inhibitor) and belatacept (a costimulation blockade agent) prolonged allograft survival in highly sensitized nonhuman primates; it appears to inhibit plasma cells, control the germinal center reaction needed to repopulate this niche, and preserve viral immunity. However, gradual rebound of donor-specific antibodies and antibody-mediated rejection suggests the need to maintain desensitization after transplantation using ongoing suppression of the B cell response. These findings suggest this mechanistic regimen may warrant further examination for translation into human protocols for desensitization, antibody-mediated rejection, and downregulating an established B cell response.

Background
Patients with broad HLA sensitization have poor access to donor organs, high mortality while waiting for kidney transplant, and inferior graft survival. Although desensitization strategies permit transplantation via lowering of donor-specific antibodies, the B cell–response axis from germinal center activation to plasma cell differentiation remains intact.

Methods
To investigate targeting the germinal center response and plasma cells as a desensitization strategy, we sensitized maximally MHC-mismatched rhesus pairs with two sequential skin transplants. We administered a proteasome inhibitor (carfilzomib) and costimulation blockade agent (belatacept) to six animals weekly for 1 month; four controls received no treatment. We analyzed blood, lymph node, bone marrow cells, and serum before desensitization, after desensitization, and after kidney transplantation.

Results
The group receiving carfilzomib and belatacept exhibited significantly reduced levels of donor-specific antibodies (P=0.05) and bone marrow plasma cells (P=0.02) compared with controls, with a trend toward reduced lymph node T follicular helper cells (P=0.06). Compared with controls, carfilzomib- and belatacept-treated animals had significantly prolonged graft survival (P=0.02), and renal biopsy at 1 month showed significantly reduced antibody-mediated rejection scores (P=0.02). However, four of five animals with long-term graft survival showed gradual rebound of donor-specific antibodies and antibody-mediated rejection.

Conclusions
Desensitization using proteasome inhibition and costimulation blockade reduces bone marrow plasma cells, disorganizes germinal center responses, reduces donor-specific antibody levels, and prolongs allograft survival in highly sensitized nonhuman primates. Most animals experienced antibody-mediated rejection with humoral-response rebound, suggesting desensitization must be maintained after transplantation using ongoing suppression of the B cell response.