COELOM-ASSOCIATED LYMPHOMYELOID TISSUE (MILKY SPOTS) - SITE OF LYMPHOID AND MYELOMONOCYTIC CELL GENERATION

Pleural and peritoneal milky spots (MS) are small morphofunctional str uctures representing subsidiary foci of coelom-associated lymphomyeloi d tissue (CALT). In this paper we studied the cellular composition of CALT in normal and Schistosoma mansoni-infected mice. In the healthy m ouse, CALT is mainly composed of IgM (+) B cells and presents lower nu mbers of CD23 and CD45R (B220) B2 lymphocytes. When activated by the i nfection, it may show pronounced lymphocytosis, plasmocytogenesis (IgM >IgG>IgA>IgG2a>IgG1) and myelomonocytosis. The lymphocytes were mainly of the B1 type (double positive CD5/IgM), with smaller number of T ce lls(TCR alpha beta(+),TCR gamma delta(+), CD3(+) and CD5(+)) and conve ntional B2 cells (B220(+),CD23(+)). The myeloid compartment was compos ed of immature and mature cells of monocyte/macrophage, eosinophil, ne utrophil and megakaryocytic lineages, especially in the omental milky spots. CALT is also a favorable microenvironment for LFA-1 (+) mast ce lls. Thus, CALT appears to be a mixed lymphoid organ, with secondary a nd/or primary lymphoid organ functions, being an important site of B1 cell generation, plasma cell maturation and extramedullar hematopoiesi s. CALT operates as an interface between blood and lymphatic circulati on and coelomic cavities, because locally or externally produced cells have easy and ready access to the pleural and peritoneal cavities. Fu rthermore, MS cells can escape into blood and lymphatic vessels, provi ding lymphocytes to other lymphoid organs and to the mucosa-associated lymphoid tissue.