Comparison of the capillary tube and explant methods for the in vitro assay of tuberculin.

Purified antigens prepared from mycobacteria were tested for nonspecific toxicity and tuberculin activity by two in vitro methods. One technique utilized measurements of macrophage migration in 4-day-old cultures of spleen from normal and tuberculin-sensitive rabbits. The other method was a modification of the capillary tube technique. To obtain enough peritoneal macrophages for good quantitation, changes were made in the method of harvesting cells, and in some instances exudates from two or more Wright strain no. 13 guinea pigs were combined. The capillary tube method was as sensitive as the explant method for detecting nonspecific toxicity. Each tuberculin assay included a group of control cultures of cells from sensitive animals, test groups containing two widely spaced concentrations of a standard tuberculin, PPD-S, and one or more concentration of the tuberculin to be tested. Macrophages from tuberculin-sensitive animals were regularly inhibited by 0.25 mug of PPD-S per ml with both in vitro methods. The potency of the test tuberculins relative to that of PPD-S was somewhat greater in capillary tube assays than in explant cultures. A purified tuberculopolysaccharide was equally inhibitory for both normal and tuberculin-sensitive cells in both culture systems.