Lymphocyte transformation to Legionella pneumophila.

Lymphocytes from seventeen subjects with previously diagnosed Legionnaires' disease and 63 subjects without a history of Legionellosis were studied in a transformation assay with a crude extract of Legionella pneumophila. In the lymphocyte transformation assay the mean stimulation index was 48+/-14 in the Legionnaires' disease group and 18+/-13 (p less than 0.001) in the control group. Endotoxin from Escherichia coli did not stimulate the cultures. Extracts from multiple other gram negative bacilli had minimal stimulation which was similar in lymphocytes from both groups. Lymphocytes from 2/3 of the subjects without a history of Legionellosis had increased incorporation of radionucleotide over background when incubated with the crude extract of L. pneumophila indicating that these subjects may have been exposed to L. pneumophila in the past and may be immune to Legionnaires' disease. This may help explain the very low attack rate (1.6-3.4%) in epidemics.