Case Report: Persistent hypogammaglobulinemia and mixed chimerism after HLA class-II disparate-hematopoietic stem cell transplant

Allogeneic hematopoietic stem cell transplantation (HSCT) is a curative treatment for various hematological, immunological and metabolic diseases, replacing the patient's hematopoietic system with donor-derived healthy hematopoietic stem cells. HSCT can be complicated by early and late events related to impaired immunological recovery such as prolonged hypogammaglobulinemia post-HSCT. We present a 16-year-old female patient with sickle-cell disease who underwent HSCT with stem cells from a human leukocyte antigen (HLA) class-II mismatched family donor. While cellular recovery was good post-HSCT, the patient developed mixed chimerism and suffered from cervical lymphadenopathy, recurrent airway infections and cutaneous SLE. She presented with hypogammaglobulinemia and was started on immunoglobulin substitution therapy and antibiotic prophylaxis. B-cell phenotyping showed that she had increased transitional and naïve mature B cells, reduced memory B cells, and diminished marginal zone/natural effector cells. In-depth immunophenotyping and B-cell receptor repertoire sequencing ruled out an intrinsic B-cell defect by expression of activation-induced cytidine deaminase (AID), presence of somatic hypermutations and differentiation into IgG- and IgA-producing plasma cells in vitro. Immunohistochemistry and flow cytometry of lymph node tissue showed a clear block in terminal B-cell differentiation. Chimerism analysis of sorted lymph node populations showed that exclusively patient-derived B cells populated germinal centers, while only a minor fraction of follicular helper T cells was patient-derived. Because of this discrepancy, we deduced that the HLA class-II disparity between patient and donor impedes facilitation of terminal B-cell differentiation in the lymph node. This case highlights disturbed cognate T-B interactions in the secondary lymphoid organs should be considered when deciphering prolonged hypogammaglobulinemia post-HSCT. Total PBMCs and B-cell subsets sorted from lymph node were used for B-cell receptor repertoire sequencing. Total RNA was isolated and amplified using BIOMED-2 primers, followed by 2x300bp sequencing on the Illumina MiSeq with the Nextera XT Library prep kit (Illumina). Raw reads were germline-allocated using IMGT and filtered for a Phred score of 25. Visualizations were created with Antigen Receptor Galaxy and Graphpad Prism and represent reads without introduced Ns and removal of duplicates based on ‘Top V, Top D, and Top J gene, identical complementarity determining region (CDR) 3-nucleotides’. IGHM reads of PBMCs were subdivided into naïve and MBCs based on having 0-to-2 or more than 2% somatic hypermutations (SHM).