Presentation Details
| Germinal center and non-germinal center B cell response to factor VIII in hemophilia A patients Maya Maarouf1, Ian Smith1, Wallace H Baldwin1, John F Healey1, Ernest T Parker1, Courtney Cox1, Robert F Sidonio, Jr1, Karen L Zimowski1, Glaivy Batsuli2, Bhavya S Doshi3, Shannon L Meeks1, Seema R Patel1. 1Aflac Cancer and Blood Disorders Center, Children’s Healthcare of Atlanta/Emory University School of Medicine, Atlanta, GA, USA.2Stanford University, Palo Alto, CA, USA.3Department of Pediatrics, Division of Hematology, Children’s Hospital of Philadelphia and University of Pennsylvania, Philadelphia, PA, USA |
Abstract
Background: Though effective at preventing bleeds, factor VIII (FVIII) mimetics do not provide persons with hemophilia A (PwHA) sufficient hemostatic activity to treat bleeds or protect during surgeries, resulting in the need for FVIII therapy. FVIII can induce inhibitors (anti-FVIII neutralizing antibodies), making clinical management of PwHA challenging. A lack of understanding regarding mechanisms that drive immunity to FVIII limits the ability to predict or prevent inhibitor formation. It has long been hypothesized that inhibitors develop through a germinal center (GC)-dependent process. However, inhibitors can be transient, or can wane or persist when FVIII is withdrawn; suggesting that the mechanisms driving inhibitor development may differ. Corroborating this, preclinical data demonstrate that inhibitors can form via a GC or non-GC process. Whether these findings extrapolate to PwHA is unknown. Thus, a pilot study that screened PwHA for formation of a GC or non-GC B cell response to FVIII was performed. Methods: Plasma and PBMCs from unique subjects enrolled in an IRB approved Inhibitor Characterization study and Biorepository from Children’s Hospital of Philadelphia were used. PwHA subjects (pediatric) included: 24 with inhibitors (PwHAi; 4 no immune tolerance induction (ITI) attempt, 4 failed ITI, 1 ongoing ITI, 15 successful ITI), 8 without inhibitors (>50 exposure days (ED) to FVIII), and 5 with <50 EDs to FVIII. Five adult healthy subjects were also included as controls. The FVIII specific B cell response was phenotyped using a FVIII B cell tetramer. The antibody response was measured by ELISA and Bethesda. Results: Within the bulk population of B cells, a greater percentage of FVIII reactive B cells was observed in all PwHA (15%) compared to healthy subjects (3%; p<0.0001). The FVIII reactive B cells in healthy subjects primarily consisted of mature B cells. Fifty-four percent of PwHA demonstrated a greater percentage (if not sole presence) of FVIII specific double negative (DN) 3 B cells (10%) that associate with non-GC responses compared to FVIII reactive class-switched memory B cells (CSW MBCs; 4%) that indicate formation of a GC response (p=0.0001). Interestingly, of the PwHA that generated a non-GC response, 16 were PwHAi (4 no ITI attempt, 1 failed ITI, 11 successful ITI), 2 were without inhibitors, and 2 had <50 ED to FVIII. The remaining PwHA demonstrated a greater percentage (if not sole presence) of FVIII specific CSW MBCs (9%) than FVIII reactive DN2/3 B cells (3%; p<0.001). Conclusion: These data suggest that immunity to FVIII can form through both a GC and non-GC process, and that non-GC responses may correlate with the ability to tolerize patients to FVIII. While these findings are from a small cohort, these results corroborate the preclinical data, support the hypothesis that differential inhibitor responses may result from engagement of distinct pathways of antibody production, and highlight the need for future studies evaluating the role of these pathways on differential inhibitor formation. Doing this may identify biomarkers for predicting which patients will develop a non-GC response (waning inhibitors), and thereby, are most likely to benefit from ITI or FVIII therapy.
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No part of this publication may be reproduced, distributed, or transmitted in any form or by any means, including photocopying, recording, or other electronic or mechanical methods, without the prior written permission of the author.