Coelom-associated lymphomyeloid tissue (milky spots): site of lymphoid and myelomonocytic cell generation

Pleural and peritoneal milky spots (MS) are small morphofunctional structures representing subsidiary foci of coelom-associated lymphomyeloid tissue (CALT). In this paper we studied the cellular composition of CALT in normal and Schistosoma mansoni-infected mice. In the healthy mouse, CALT is mainly composed of IgM (+) B cells and presents lower numbers of CD23 and CD45R (B220) B2 lymphocytes. When activated by the infection, 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 cells (TCR alpha beta (+), TCR gamma delta (+), CD3 (+) and CD5 (+)) and conventional B2 cells (B220 (+), CD23 (+)). The myeloid compartment was composed of immature and mature cells of monocyte/macrophage, eosinophil, neutrophil and megakaryocytic lineages, especially in the omental milky spots. CALT is also a favorable microenvironment for LFA-1 (+) mast cells. Thus, CALT appears to be a mixed lymphoid organ, with secondary and/or primary lymphoid organ functions, being an important site of B1 cell generation, plasma cell maturation and extramedullar hematopoiesis. CALT operates as an interface between blood and lymphatic circulation and coelomic cavities, because locally or externally produced cells have easy and ready access to the pleural and peritoneal cavities. Furthermore, MS cells can escape into blood and lymphatic vessels, providing lymphocytes to other lymphoid organs and to the mucosa-associated lymphoid tissue.