Oxytocin-producing and vasopressin-producing eosinophils in the mouse spleen: immunohistochemical, immuno-electron-microscopic and in situ hybridization studies.

Oxytocin-like and vasopressin-like immunoreactive cells, and the cells expressing mRNAs for these peptides in the spleen of the C57BL/6 mouse were studied by immunohistochemistry, immuno-electron microscopy and in situ hybridization. Immunoreactive cells were distributed mainly in the splenic cord and marginal zone, whereas there were few in the lymphocyte-packed periarteriolar-lymphoid sheath, lymphoid follicle and germinal center. More numerous vasopressin-positive cells were seen in the splenic cord. The colocalization of oxytocin-like and vasopressin-like immunoreactivity in the same cells was identified by the investigation of mirror sections. By the pre-embedding immuno-electron-microscopic method using antisera against oxytocin and vasopressin, immunopositive reaction products were localized in the matrix around the specific granules, small clear vesicles and mitochondrial membrane of the eosinophils. No immunoreactivity to these peptides was found within the specific granules of the eosinophils. In situ hybridization with synthetic oligonucleotide probes labeled with 32P revealed the presence of mRNAs for oxytocin and vasopressin in the cells of the spleen, the distribution of the mRNAs for these peptides being the same as that of immunopositive cells. These observations suggest that eosinophils synthesize both oxytocin and vasopressin and store them in the matrix. Possible differences in the mechanism of synthesis and storage of these peptides between peripheral eosinophils and hypothalamic neurons are discussed.