Characterization of Lymphocyte-Activating Factor (LAF) Produced by Human Mononuclear Cells: Biochemical Relationship of High and Low Molecular Weight Forms of LAF

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Abstract
Human mononuclear cells (MNL) stimulated with Salmonella typhosa endotoxin (LPS), Actinomyces viscosus cell wall extracts, phorbol myristic acetate (PMA), or phytohemagglutinin (PHA) produced a factor that was mitogenic for mouse thymocytes. These agents not only enhanced the production of this mitogenic factor(s) by cultures of MNL enriched for glass-adherent cells, but also by nonadherent cell cultures. However, when the monocyte contamination of the nonadherent cell population was decreased to less than 0.5%, the remaining nonadherent cells did not generate significant thymocyte mitogenic activity, suggesting that either the contaminating monocytes were the cell source of the factor in the nonadherent cell population or that the production of the factor by nonadherent cells was monocyte dependent. The results of biochemical studies support the former possibility as follows: 1). The mitogenic activity produced by both adherent and nonadherent MNL eluted from Bio-Gel P-100 columns with m.w. of 50,000 to 70,000 and 12,000 to 22,000, values that are characteristic of the two forms of lymphocyte activity factor (LAF) produced by human monocytes. 2). The lower m.w. form of “thymocyte mitogenic” activity produced by the adherent as well as nonadherent MNL when characterized further both exhibited the same behavior on Bio-Gel P-100, DEAE cellular and hydroxylapatite. 3). When the lower m.w. LAF was reconstituted with 2% human serum, concentrated and rechromatographed on Bio-Gel P-100, a significant portion of the LAF activity appeared in the 50,000 to 70,000 m.w. range, suggesting that the higher m.w. form of LAF may actually be a complex of the lower m.w. LAF with a human serum component(s). In conclusion, a small population of contaminating monocytes in a nonadherent MNL population is capable of producing significant amounts of LAF, and LAF may exist free or in association with a component(s) of human serum.