Functional communication between the endocrine and immune system at the pituitary: Pleiotropism or transient differentiation stages within the folliculo-stellate and dendritic cell group.

Functional communication between the endocrine and immune systems has been described in terms of the common expression of signalling molecules, receptors and coupling proteins. However, information transfer in immune-endocrine interactions may also result from transgression of diffusion barriers in the body by immune effector cells. The presence of myeloid cells in the anterior pituitary (AP) of mouse and rat under non-pathological conditions, illustrates the importance of immune accessory cells in immune-endocrine interactions in the AP. Using double immunogold-labelling of ultrathin cryo-sections of rat AP, we demonstrate the presence of MHC class II+ S100(+) cells with ultrastructural characteristics of pituitary folliculo-stellate (FS) and dendritic cells (DC). Our data suggest the heterogeneity of the group of S100(+) FS cells, in that this group may contain a subpopulation of lymphoid DC, which are also present in other endocrine organs. Moreover, our data indicate that dispersed cell populations of the rat AP contain a fraction of immunocompetent cells, that are capable of stimulating T lymphocyte proliferation in vitro. The latter cell fraction is also enriched in S100(+) and MHC class II+ cells. Furthermore, in the present study a review is given of the problem of the nomenclature of pituitary folliculo-stellate cells and dendritic cells, whereby special attention is given to the quite different ways in which both cell types are defined.