The relationship between complement and the heat labile factors known to be necessary for phagocytosis of Cryptococcus neoformans by human leukocytes has been examined. C. neoformans or purified cryptococcal polysaccharide consumed 36.2 to 76.3% of late complement components (C3-9) in guinea pig serum. Phagocytic studies with C2-deficient human serum and C4-deficient guinea pig serum permitted evaluation of the roles of the classical (C142) and alternate complement pathways in the phagocytosis of these organisms.

The number of cryptococci ingested per neutrophil (phagocytic index) at 30 min in sera with only alternate pathway function was 40 to 60% of that found in sera with both classical and alternate complement pathways intact. When purified C2 or C4 was added back to the corresponding deficient sera, phagocytic indices increased to ranges obtained by using corresponding normal sera with both complement pathways active. By 2 hr, phagocytic indices for deficient sera equaled phagocytic indices for normal sera. Early classical complement components were therefore required for optimum phagocytic kinetics. The fact that late components of complement were required for phagocytosis was established by the fact that chelation of Ca⁺⁺ and Mg⁺⁺ with EDTA inhibited the reaction, as did heating at 56°C and use of serum depleted of the late components of complement by cobra venom factor. None of the sera used in these studies had detectable anti-cryptococcal antibody by an indirect fluorescent antibody technique, but absorption of human sera at 0°C with cryptococci lowered phagocytic indices to about 10% of normal. The loss of activity after absorption was demonstrated to be due to loss of specific antibody which was required to activate the C142 pathway as well as to loss of properdin which was required to activate the alternate pathway. The classical complement pathway appeared to be functioning primarily to activate the alternate pathway, which in turn was responsible for opsonization. Serum depleted of properdin or of the heat labile properdin factor B was markedly deficient in opsonic activity. In the former case addition of properdin restored optimal kinetics. Thus, function of the classical pathway in normal serum did not adequately mediate opsonization.

Unlike serum, spinal fluid did not opsonize cryptococci. With fluorescent anti-human β_1C, little or no complement was detectable on the surface of cryptococci obtained from the spinal fluid of four patients with cryptococcal meningitis, but was found on cryptococci after incubation in fresh sera from these patients. However, cerebrospinal fluid from two of three patients with active disease could restore opsonic activity of fresh serum absorbed with cryptococci.