CHARACTERIZATION OF HUMAN FIBROLEUKIN, A FIBRINOGEN-LIKE PROTEIN SECRETED BY T-LYMPHOCYTES

We have recently cloned the human homologue of the murine pT49 cDNA (( hpT49h), a transcript encoding a protein homologous to the beta- and g amma-chains of fibrinogen, Here, we report alae identification of the hpT49h gene product using mAbs generated against a peptide correspondi ng to the carboxyl-terminal end of the deduced protein and a recombina nt protein fragment expressed in Escherichia coli. mAbs 23A6, 7B12, an d 3F4 specifically recognized a protein of 70 kDa in reducing SDS-PAGE in the culture supernatant of 293T cells transiently transfected with the full length hpT49h cDNA and freshly isolated PBMC, Under nonreduc ing conditions, the material migrated with a molecular mass of 250 to 300 M)a, indicating that the 70-kDa protein forms a disulfide bonded c omplex. Because of its homology with fibrinogen, we have termed this p rotein fibroleukin. Fibroleukin is spontaneously secreted in vitro by freshly isolated CD4(+) and CD8(+) T lymphocytes. RT-PCR analysis reve aled preferential expression of fibroleukin mRNA in memory T lymphocyt es (CD3(+)/CD45R0(+)) compared with naive T lymphocytes (CD3(+)/ CD45R A(+)). Fibroleukin production by PBMC was rapidly lost in culture. Pro duction could he partially maintained in the presence of IFN-gamma, wh ile T lymphocyte activation had no effect. To demonstrate fibroleukin production in vivo, we analyzed colon mucosa by immunohistology. Fibro leukin staining was detected in the extracellular matrix of the T lymp hocyte-rich upper portion of the lamina propria mucose. While the exac t function of fibroleukin remains to be defined, these data suggest th at fibroleukin may play a role in physiologic lymphocyte functions at mucosal sites.