Phylogeny of lymphocyte heterogeneity: the cellular requirements for in vitro mitogenic responses of channel catfish leukocytes.

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Abstract
Cell separation and enrichment techniques were employed to isolate three distinct leukocyte subpopulations present in channel catfish (Ictalurus punctatus) peripheral blood. Surface immunoglobulin-positive (sIg+) and sIg- lymphocytes were separated by an indirect "panning" technique employing monoclonal antibodies reactive with channel catfish Ig. A third cell population composed of macrophages was isolated by adherence to baby hamster kidney cell microexudate-coated surfaces. Functional features of these three subpopulations were assessed by in vitro mitogenic responses to lipopolysaccharide (LPS) and concanavalin A (Con A). The results that were obtained indicated that the sIg+ cells responded only to LPS stimulation regardless of the presence or absence of macrophages. The sIg- subpopulation, however, responded to neither LPS nor Con A unless macrophages were present, in which case responses were obtained to both mitogens. The accessory cell nature of the macrophages was shown by experiments utilizing fixed numbers of one cell type mixed with varying numbers of another cell type. Furthermore, the accessory cell function was abrogated by passage through Sephadex G-10 and preincubation with L-leucine methyl ester. These studies provide further evidence that teleosts not only contain B and T cells akin to those in mammalian systems, but contain accessory cells (macrophages) as well.