Background: Anti-Gal antibodies cause hyperacute and delayed xenograft rejection in pig-to-primate transplantation. The cell populations producing anti-Gal and other natural antibodies in primates are unknown.
Methods: Cells from different lymphoid compartments of naïve or sensitized baboons were examined for anti-Gal and total Ig production by ELISPOT. B and plasma cells from humans and baboons were purified by FACS sorting and characterized for anti-Gal and total Ig production and cytology.
Results: In naïve baboons, the spleen was the major source of anti-Gal IgM-secreting cells. Two months after sensitization with porcine tissues, high frequencies of anti-Gal IgM- and IgG-secreting cells were detected in the spleen, lymph nodes, and bone marrow. Six months after antigen exposure, anti-Gal IgM- and IgG-secreting cells were preferentially localized in the bone marrow. Cells from human spleen, bone marrow, and blood were also analyzed and anti-Gal IgM-secreting cells were detected mainly in the spleen. Sorting of baboon and human cells showed that anti-Gal IgM-secreting cells were mainly splenic B cells (CD20+, CD138-, and Ig+). Although low in percentage, sorted CD20-CD138+ plasma cells in spleen and bone marrow secreted large quantities of anti-Gal IgM. Most anti-Gal IgG-secreting cells were plasma cells (CD138+) at both early (Ig+) and late (Ig-) stages of differentiation.
Conclusions: Similar to Gal knockout mice, natural anti-Gal IgM antibodies in primates are produced mainly by splenic B cells. After antigen exposure, anti-Gal IgM and IgG were secreted by both B and plasma cells. These results suggest strategies to remove xenoreactive antibody-secreting cells prior to transplantation.