Role of platelet factors and serum complement in growth of fibroblasts with high-affinity Clq complement receptors

Cultures of human diploid fibroblasts are heterogeneous in that a subpopulation interacts via high-affinity receptors with the globular head regions of the Clq complement protein. Growth and synthetic properties of these cells are characteristic of cells residing in healing wounds and inflammatory lesions. At these sites, fibroblasts are exposed to regulatory molecules such as complement components and factors released from blood platelets. We assessed the effects of native complement proteins and platelet-derived factors on proportions and phenotypic stability of high-affinity and low-affinity receptor cells generated from explants of adult and embryonic connective tissue, using radioligand binding assays and immunofluorescence analysis by flow cytometry. Fibroblasts expressing high-affinity Clq receptors could be generated from explants only when factors from platelets were present in the medium; native complement proteins were not essential. High-affinity receptor cells could be generated only from tissue; they could not be generated by incubating cultures of the low-affinity receptor phenotype in medium containing platelet-derived factors. High-affinity receptor cells, once established from explants in the presence of platelet-derived factors, persisted through many replications in the absence of platelets. We obtained the same fibroblast phenotypes from embryonic skin as from adult gingiva, but the proportion of high-affinity receptor cells from skin was much greater. We conclude that factors derived from platelets are essential for generating cultures containing fibroblasts expressing high-affinity Clq receptors, but not for their maintenance. High-affinity receptor cells may comprise a rapidly dividing subpopulation giving rise only to like progeny or to other, more differentiated cells.