We have previously reported that virulent egg yolk-grown Legionella pneumophila, Philadelphia 1 strain, multiplies intracellularly in human blood monocytes. We now report on the interaction between virulent L. pneumophila and human polymorphonuclear leukocytes (PMN), antibody, and complement, in vitro, under antibiotic-free conditions.

L. pneumophila in concentrations ranging from 10(3) to 10(6) colony forming units (CFU)/ml are completely resistant to the bactericidal effects of 0-50 percent fresh normal human serum, even in the presence of high concentrations of rabbit or human anti-L. pneumophila antibody.

L. pneumophila bacteria fix the third component of complement (C3) to their surfaces, as measured by fluorescence microscopy using rhodamine- conjugated goat anti-human C3 IgG, only when the bacteria are incubated with both specific anti-L. pneumophila antibody and complement. Similarly, L. pneumophila adhere to PMN, as measured by fluorescence microscopy, only in the presence of both specific antibody and complement. Electron microscopy revealed that these opsonized bacteria are phagocytosed by the PMN.

PMN require both antibody and complement to kill L. pneumophila; even then, PMN reduced CFU of L. pneumophila by only 0.5 log under conditions in which they reduce CFU of a serum-resistant encapsulated strain of Escherichia coli by 2.5 logs. Separation of PMN-associated and nonassociated CFU of L. pneumophila revealed that the major proportion of the surviving bacteria are PMN associated. Thus, the ineffective killing of opsonized L. pneumophila is a result of a failure of PMN to kill these bacteria after they become PMN- associated. With or without antibody, PMN do not support the growth of L. pneumophila. These findings suggest that PMN, even in conjunction with the humoral immune system, do not play a decisive role in defense against the Legionnaires' disease bacterium.

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