Opsonization of pneumococci. II. Metabolic effects of a "third" human serum activity that mediates intracellular killing.

We investigated the mechanisms by which a serum activity, neither complement nor immunoglobulin, mediates killing of pneumococci by polymorphonuclear leukocytes (PMN). Electron microscopy revealed type 25 pneumococci to be within PMN when incubated in normal serum, in serum absorbed twice at 0 degrees C with type 25 pneumococci, or in absorbed plus heat-inactivated serum. Uptake of radiolabeled bacteria, and activation of oxygen consumption and of the hexose monophosphate shunt by PMN with pneumococci, were similar in normal serum, absorbed serum, or the combination of absorbed and heat-activated serum. Reduction of nitroblue tetrazolium (NBT) and of cytochrome c by PMN in the presence of type 25 pneumococci and absorbed serum, with or without heat-inactivated serum were one-third and one-half, respectively, of those in normal serum. Likewise, protein iodination was one-half that in normal serum. Reduction of cytochrome c by cytochalasin B-treated PMN was the same in normal, absorbed, or absorbed plus heat-inactivated serum. Furthermore, release of beta-glucuronidase from PMN after ingestion of pneumococci in 10% normal, absorbed, or absorbed plus heat-inactivated serum was identical. These data indicate that the "third" serum activity is not necessary for attachment of pneumococci to or ingestion by PMN, nor is it necessary for stimulation of the plasma membrane oxidase. Rather, it functions somehow in intracellular killing.