Postnatal development of T cells in dogs with X-linked severe combined immunodeficiency

X-linked severe combined immunodeficiency disease (XSCID) in both humans and dogs results from mutations in the common γ-chain, γc, which is a common component of the receptors for IL-2, IL-4, IL-7, IL-9, and IL-15. Although human and canine XSCID share similar features, such as a failure to thrive, hypogammaglobulinemia, an absent T cell mitogenic response, and thymic dysplasia, near normal percentages of T cells are observed in some affected dogs, whereas XSCID boys have few, if any, circulating T cells. In this study, PBL were analyzed by flow cytometry beginning shortly after birth until 9 wk of age. XSCID dogs <3 wk of age had an elevated number of B cells and were nearly devoid of T cells, phenotypically resembling most human XSCID patients. At 5 wk of age, however, T cells appeared in approximately one- half of the XSCID dogs, although the absolute number of T cells was one- third of normal in these dogs. While the percentage of CD45RA⁺ T cells in normal dogs gradually decreased with age from >90% in neonates to <40% by 3 to 5 yr of age, in XSCID dogs a rapid decline in the percentage of CD45RA⁺ T cells was observed, resulting in <10% CD45RA⁺ T cells by 7 to 9 wk of age. Maternal engraftment was not detected in any of the XSCID dogs by using a sensitive PCR assay. The appearance of nonmaternally derived T cells in XSCID dogs that undergo a rapid switch from CD45RA⁺ to CD45RA^- suggests that limited thymic emigration and peripheral expansion of T cells can occur in the absence of a functional γc.