Purpose Severe combined immunodeficiency (SCID) is a syndrome of diverse genetic cause characterized by profound deficiencies of T, B and sometimes NK cell function. Non-ablative HLA-identical or rigorously T cell-depleted haploidentical parental bone marrow transplantation (BMT) results in thymus-dependent genetically donor T cell development in the recipients, leading to a high rate of long-term survival. However, the development of B cell function has been more problematic. We report here results of analyses of B cell function in 125 SCID recipients prior to and long-term after non-ablative BMT, according to their molecular type. Methods Studies included blood immunoglobulin measurements; antibody titers to standard vaccines, blood group antigens and bacteriophage Φ × 174; flow cytometry to examine for markers of immaturity, memory, switched memory B cells and BAFF receptor expression; B cell chimerism; B cell spectratyping; and B cell proliferation. Results The results showed that B cell chimerism was not required for normal B cell function in IL7Rα-Def, ADA-Def and CD3-Def SCIDs. In X-linked-SCID, Jak3-Def SCID and those with V-D-J recombination defects, donor B cell chimerism was necessary for B cell function to develop. Conclusion The most important factor determining whether B cell function develops in SCID T cell chimeras is the underlying molecular defect. In some types, host B cells function normally. In those molecular types where host B cell function did not develop, donor B cell chimerism was necessary to achieve B cell function. 236 words

Severe combined immunodeficiency (SCID) is a syndrome of diverse genetic cause characterized by profound deficiencies of T, B, and sometimes NK-cell function. Nonablative human leukocyte antigen-identical or rigorously T cell-depleted haploidentical parental bone marrow transplantation (BMT) results in thymus-dependent genetically donor T-cell development in the recipients, leading to long-term survival. We reported previously that normal T-cell numbers, function, and repertoire developed by 3 to 4 months after transplantation in SCID patients, and the repertoire remained highly diverse for the first 10 years after BMT. The T-cell receptor diversity positively correlated with T-cell receptor excision circle levels, a reflection of thymic output. However, the fate of thymic function in SCID patients beyond 10 to 12 years after BMT remained to be determined. In this greater than 25-year follow-up study of 128 patients with 11 different molecular types of SCID after nonconditioned BMT, we provide evidence that T-cell function, thymic output, and T-cell clonal diversity are maintained long-term.

CD3ζ is a subunit of the T-cell antigen receptor (TCR) complex required for its assembly and surface expression that also plays an important role in TCR-mediated signal transduction. We report here a patient with T−B+NK+ severe combined immunodeficiency (SCID) who was homozygous for a single C insertion following nucleotide 411 in exon 7 of the CD3ζ gene. The few T cells present contained no detectable CD3ζ protein, expressed low levels of cell surface CD3ε, and were nonfunctional. CD4+CD8−CD3εlow, CD4−CD8+CD3εlow, and CD4−CD8−CD3εlow cells were detected in the periphery, and the patient also exhibited an unusual population of CD56−CD16+ NK cells with diminished cytolytic activity. Additional studies demonstrated that retrovirally transduced patient mutant CD3ζ cDNA failed to rescue assembly of nascent complete TCR complexes or surface TCR expression in CD3ζ-deficient MA5.8 murine T-cell hybridoma cells. Nascent transduced mutant CD3ζ protein was also not detected in metabolically labeled MA5.8 cells, suggesting that it was unstable and rapidly degraded. Taken together, these findings provide the first demonstration that complete CD3ζ deficiency in humans can cause SCID by preventing normal TCR assembly and surface expression.

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