DIFFERENTIAL THY-1 EXPRESSION BY SPLENIC FIBROBLASTS DEFINES FUNCTIONALLY DISTINCT SUBSETS

Fibroblasts have an important structural role in the spleen, as they p rovide a scaffold of extracellular matrix in which cells of the immune system reside. Aside from their vague recognition as ''stromal'' or ' 'reticular'' components of the spleen, these cells have not beers char acterized. In this study, normal fibroblast nines from mouse [B6D2(F-1 )] spleen were established. The fibroblast phenotype of these lines wa s confirmed by their morphology, expression of vimentin, as well pas t heir lack of epithelial and endothelial cell markers, their failure to display the hematopoietic marker CD45, and their inability to phagocy tize. Interestingly, 50-65% of the selenic fibroblasts expressed the T hy-1 antigen, while a subpopulation of Thy-1-negative fibroblasts exis ted. FACS on the basis of Thy-1, as well as limiting dilution cloning, yielded stable lines and clones of Thy-1(+) and Thy-1(-) splenic fibr oblasts. Phenotypic characterization revealed that both subsets synthe sized collagen and expressed class I MHC, ICAM-1, VCAM-1, and CD44 con stitutively. However, intriguing differences existed between the fibro blast subpopulations. Thy-1(+) splenic fibroblasts produced significan tly greater levels of IL-6 than did their Thy-1 counterparts. After tr eatment with IFN-gamma (150 U/ml, 72 hr), Thy-1, but not Thy-1(+), spl enic fibroblasts expressed class II MHC and presented antigen to an I- A(b)-restricted T cell line. This suggests that the Thy-1 fibroblasts may present antigen to T lymphocytes in vivo under inflammatory condit ions. Thus, splenic fibroblasts are a heterogeneous and dynamic cell t ype poised in an immunologically relevant location to interact with bo ne marrow-derived cells under normal and fibrotic conditions. (C) 1996 Academic Press, Inc.