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The migration inhibitory activity of supernatants of Con A-Sepharose stimulated MNC cultures was tested by measuring the migration of [3H]leucine-labeled granulocytes or macrophages from agarose-containing capillary tube fragments. The migrated cells were harvested with an automated cell harvester on glass fiber filters and counted in a beta counter. Suspension of the target cells in agarose improves the accuracy of the method since migration from packed capillary tube fragments is sometimes not significantly different as between control and assay preparations. The concentration of agarose may inversely influence results, lower concentrations correlating with higher migration inhibition in the presence of assay supernatants. Different migration inhibition results are obtained when allogeneic granulocytes are used in parallel. For this reason a 10 times concentrated supernatant from a Con A-Sepharose-stimulated MNC culture, exhibiting the highest LIF activity at any mitogen concentration, was used as standard considered arbitrarily to contain 100 U. On the parallel regression lines of the dilution curves of the standard and assay preparations, reference with the standard is made at 25% migration inhibition as the comparison point.

Soluble membrane fractions derived from Raji cells trigger lymphocytes of Epstein-Barr virus (EBV)-seropositive, but not EBV-seronegative, individuals to release a lymphokine that inhibits leukocyte migration. The reaction can be blocked by the sera of patients with EBV-DNA-carrying tumors, Burkitt lymphoma, or nasopharyngeal carcinoma. Absorption of these sera with EBV-positive, but not EBV-negative, cells abrogates their blocking activity. These findings suggest that the antigen responsible for the leukocyte migration inhibition reaction is an EBV-encoded or an EBV-induced membrane component. The antigen is not identical with EBV-associated nuclear antigen or any other known antibody-detected EBV antigen.

Migration inhibitory factor (MIF) was produced by lymphoid cells of Lewis rats in response to infection with Bacille Calmette Guérin (BCG), mixed lymphocyte reaction or concanavalin A stimulation. It was found that rat lymphokine-containing supernatants demonstrate a definite species specificity, in that MIF activity is observed only when rat indicator cells and not when guinea-pig indicator cells are used. A comparative study of MIF activities, using rat peritoneal exudate cells, revealed that all three methods stimulated MIF production by either spleen or lymph node cells. Only infection with BCG induced peripheral blood leukocytes to elaborate MIF. In all cases, rat MIF inhibited rat peritoneal exudate cells but not guinea-pig peritoneal exudate cells. Possible explanations for the species specificity of rat MIF including a role for receptor carbohydrate moieties and fibronectin are discussed.