Leukin, a Bactericidal Agent from Rabbit Polymorphonuclear Leucocytes

Abstract

THE term leukin was proposed almost 60 years ago to denote a class of granulocytic antibacterial agents selectively active against Gram positive bacteria¹. During the past decade, sufficient progress has been made to indicate the potential significance in host defence of such bactericidal agents. Knowledge of the chemical nature of a potent leukin present in rabbit neutrophiles first emerged in 1956. On the basis of high arginine (17 per cent) and low lysine (4.5 per cent) content, this leukin was considered to be a protamine derivative². It was subsequently shown that bacteria undergoing dissolution within developing inflammatory lesions become progressively coated with a highly basic arginine-positive substance³, which is probably delivered to the bacterial surface during phagocytosis. Leucocytic granules, which contain the bulk of the bactericidal agent⁴, appear to fuse with phagocytic pouches containing bacteria and to release their contents into these pouches during degranulation^5,6. Such a highly localized intracellular delivery system is probably most important because highly basic proteins or polypeptides would be neutralized and rendered less effective in an extracellular milieu. It is interesting that granulocytes from patients with an X-linked granulomatous disease seem to phagocytose normally but do not kill ingested bacteria⁷. The leucocytic defect is apparently related to improper degranulation rather than to a deficiency in the bactericidal agent⁸.