Role of de Novo Protein Synthesis by Human Mononuclear Leukocytes in Opsonin-Independent Phagocytosis

Human peripheral blood lymphocytes or the soluble (105,000g) supernatant of the lymphocyte lysate can increase the percentage of human monocytes ingesting particulate activators of the human alternative complement pathway, e.g., rabbit erythrocytes (Chakravarti et al., J. Immunol. 137, 880, 1986). We now show that preincubation of the lymphocytes led to an increase in their augmenting ability. However, no such increase in the augmenting ability of the lymphocytes or soluble supernatant made from these lymphocytes was observed when they were preincubated and added to monocytes in the presence of cycloheximide; rather there was a significant reduction in the ingesting ability or the monocytes. Our results demonstrate that in the case of intact lymphocytes, the observed inhibition was because of (i) inhibition of de novo synthesis of protein(s) in the monocytes during their adherence and (ii) inhibition of de novo synthesis of protein(s) in the lymphocytes, possibly of a signaling molecule necessary for release of the peptide cytokine, phagocytosis-inducing factor (PIF), from the lymphocytes. However, the inhibition observed with soluble supernatant made from lymphocytes preincubated in the presence of cycloheximide was because of the above phenomenon (i) only and indicated that there was no de novo synthesis of PIF during preincubation of the lymphocytes.