Transplantation of hematopoietic stem cells into fetal sheep early in gestation (day (d) 55) (term gestation = 145d) results in long-term multi-lineage hemopoietic chimerism. We have proposed that the fetus is immunologically receptive at this time. However, our understanding of the mechanisms underlying this establishment of fetal tolerance remains poorly defined. We hypothesized that examination of developmental events in fetal immune ontogeny might provide insights into mechanisms underlying fetal tolerance formation. To this end we have previously reported the early appearance of cells phenotypically consistent with T regulatory cells (CD4+/CD25+) and multilineage expression of the CD45 isoform CD45R early in gestation (d40–65). Here, we extend our observations on B lineage cell distribution and relative CD45 isoform expression during gestation. The two commercially-available ovine CD45 antibodies are CD45 (immunoprecipitates proteins of MW 225, 210, and 190 kD, (

Immunology.
1985
;
55
:
347
–353
)) and CD45R (immunoprecipitates a single protein of MW 220 kD (
Cell. Immunol.
1987
;
110
:
46
–55
)). Using dual-color flow cytometry, cells from fetal thymus, intestine, lung, spleen, bone/bone marrow, and peripheral blood (PB) were analyzed for expression of surface IgM, CD45 and CD45R from d39 to two days after birth. Surface IgM+ cells in the thymus were observed to be as high as 28% at d39 of gestation (uniformly CD45R+/CD45−). This rapidly dissipated to less than 1% after d65 of gestation, although IgM+ cells were observed as late as two days after birth. All IgM+ cells expressed CD45R in all organs tested but relative CD45 expression varied by gestational age and organ. Subsequent sampling noted rapid onset of CD45 coexpression on >70% of detectable thymic surface IgM+ cells throughout gestation. Similarly, splenic IgM+ cells co-expressed high levels of CD45 while PB IgM+ cells had delayed CD45 expression (beyond d65). Bone/bone marrow showed variable coexpression of CD45 throughout gestation. By contrast, surface IgM+ cells in lung and intestine maintained CD45R expression without coexpression of CD45 prior to birth. We have previously reported humoral tolerance to retroviral vector transgene products transplanted at d55–60 (
Blood.
2001
;
97
:
3417
–3423
). Further understanding of CD45 isoform expression patterns and relative organ distribution of surface IgM+ cell populations during the time of immune receptivity in the sheep may provide insight into fetal transplantation tolerance.

Topics:
b-lymphocytes, cd45 antigens, fetus, protein isoforms, transplantation tolerance, kawasaki's disease, ketogenic diet, antibodies, flow cytometry, immunoglobulin m

Disclosure: No relevant conflicts of interest to declare.

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2006, The American Society of Hematology
2006
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