Enzymatic modifications of human plasma fibronectin in relation to opsonizing activity

Plasma fibronectin is one of the largest plasma proteins (Mr approximately 440 000), comprising two approximately equal polypeptide chains which are held together by a disulfide linkage near the C-terminal end of the molecule. The binding of gelatinized latex beads to liver slices as well as the internalization of these particles by macrophages, in the presence of heparin, is greatly enhanced by fibronectin. The question as to whether the entire covalent structure of fibronectin was necessary for opsonizing activity was approached by limited proteolytic degradations of the molecule. Patterns of controlled digestion with trypsin, cathepsin D, Staphylococcus aureus protease, and plasmin all indicate that the minimal unit necessary for retention of opsonic activity is some large (Mr 200 000 and 190 000) single-chain entity. Treatment with plasmin proved to be the most reliable procedure for generating the active split product which could be readily separated from the inactive, disulfide-containing C-terminal fragment. Incorporation of dansylcadaverine into plasma fibronectin (3.5 mol/mol of protein) by fibronoligase (coagulation factor XIIIa) did not affect the opsonic activity of the protein.     

Antigen-specific T helper clones in a nonresponder strain require augmentation for expression of helper activity. Evidence for a possible antigen presentation defect in B cells

Hen egg-white lysozyme (HEL)-specific Thy-1+, Lyt-1+2- T cell lines and clones were derived from the nonresponder C57BL/6 strain. Although the antigen-specific proliferative response of these T cells in the presence of syngeneic irradiated spleen cells as a source of antigen-presenting cells (APC) was normal, the same cells were incapable of stimulating B cells to secrete antibody in vitro. This deficiency could, however, be corrected by the addition of an excess of normal T cells or a supernatant from concanavalin A-stimulated rat spleen cells. Alternatively, the use of highly cross-reactive ring-necked pheasant lysozyme in the cultures allowed expression of efficient help, ruling out any inherent deficiency in the T cells. The antibody response was specific and required MHC compatibility between the T lines and responding B cells. By using (H-2b X H-2d)F1 B cells and another H-2d-restricted HEL-specific T line, it was shown that only the H-2b-restricted T-B collaboration required exogenous factors, and the H-2d-restricted collaboration did not. Because both proliferative and helper responses are dependent upon MHC-restricted antigen presentation by macrophage-APC and B cells, respectively, these results suggest that the defect in the nonresponder H-2b-restricted T-B collaborative pathway may relate to the inability of B cells to adequately process and present HEL to clonal T cells.     

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