HETEROGENEITY OF PITUITARY FOLLICULO-STELLATE CELLS - IMPLICATIONS FOR INTERLEUKIN-6 PRODUCTION AND ACCESSORY FUNCTION IN-VIVO

The population of folliculo-stellate (FS) cells of the rat anterior pi tuitary has been shown to be ultrastructurally and immunohistochemical ly heterogeneous. Based on the overlap of ultrastructural characterist ics, the localization in the anterior pituitary and the co-expression within the same eel of the S-100 protein (a marker for FS cells) and M HC-class II determinants (an immune marker) we concluded that a partia l overlap exists between the population of FS cells and the monocyte-d erived dendritic cells (DC). In this report we describe that interleuk in-6 (IL-6) immunoreactivity was found in situ in stellate cells of th e rat, mouse and human anterior pituitary at a very low density (<1% o f the cells); the topography was reminiscent of the distribution of FS cells. In the present study we also analyse three different pituitary cell separation methods, in order to study the functional heterogenei ty of the FS cells in vitro, and to verify whether functionally distin ct subpopulations exist within the FS cell group. Production of bioact ive IL-6 was measured in conditioned media of rat anterior pituitary c ells separated by (i) bovine serum albumin (BSA) gradient sedimentatio n at 1 g, (ii) Nycodenz gradient and (iii) a magnetic cell separation (MACS) technique. Production of bioactive IL-6 by cell cultures of 1 t o 4 days was correlated with the proportional number of S100 immunorea ctive and S100 producing cells, but was not correlated with the propor tional number of MHC-class II expressing (OX6-positive) dendritic cell s (DC). The distribution pattern of OX6-positive DC was found to partl y overlap with the distribution pattern of S100-positive cells in the BSA gradient. Co-sedimentation of S100-positive FS cells and MHC-class Ii-expressing DC was not restricted to the top fractions of the BSA g radient, but was also found in the low density Nycodenz fraction. MACS separation of the rat anterior pituitary cells resulted into a popula tion enriched in OX6 and OX62 positive DC and a population devoid of s uch cells, while S100(+) cells were equally divided into these two sub populations. Although there was a significantly decreased production o f IL-6 as compared to that of an original pituitary cell population, b oth MACS separated populations were equal in IL-6 production. The dimi nution in IL-6 production in both populations may be the result of an impediment of paracrine communication due to the MACS separation into these two populations. Our data also show that a subpopulation of FS c ells was capable of stimulating T cell proliferation in vitro. Concomi tantly with the distribution pattern of S100- and OX6-immunoreactive c ells in the BSA and Nycodenz gradient fractions, we found a similar pa ttern of stimulation of T cell proliferation. Unlike the IL-6 producti on pattern, the T cell stimulating capacity was present in the MHC-cla ss Ii-enriched cell population but absent in the MHC-class II-depleted cell population. These findings-together with earlier in situ histoch emical data-suggest that there is an OX6(+) S100(+) subpopulation of F S cells in the anterior pituitary that in itself is capable of stimula ting T cell proliferation in vitro, and acts as lymphoid DC. There is also an S100(+) OX6(-) population that is unable to stimulate T cell p roliferation in vitro. Both populations are able to produce IL-6, but probably need stimuli from other subpopulations of pituitary cells (or exogeneous stimuli) to produce maximal amounts of IL-6.