Analysis of naturally occurring delayed-type hypersensitivity reactions in leprosy by in situ hybridization

Analysis of tissue lesions of the major reactional states of leprosy was undertaken to study the immune mechanisms underlying regulation of cell-mediated immunity and delayed-type hypersensitivity (DTH) in man. In situ hybridization hybridization of reversal reaction biopsy specimens for INF-γ mRNA expression revealed a 10-fold increase in specific mRNA-containing cells over that observed in unresponsive lepromatous patients. Expression of huHF serine esterase, a marker for T cytotoxic cells, were fourfold increased in reversal reaction and tuberculoid lesions above that detected in unresponsive lepromatous individuals. Immunohistology of reversal reactions confirmed a selective increase of Th and T cytotoxic cells in the cellular immune response. Of interest, the microanatomic location of these serine esterase mRNA-containing cells was identical to the distribution of CD4⁺ cells. Analysis of erythema nodosum leprosum (ENL) lesions revealed differences in the underlying immune processes in comparison with reversal reaction lesions. Although phenotypic Th cells predominated in ENL lesions, IFN-γ and serine esterase gene expression were markedly reduced. We suggest that reversal reactions represent a hyperimmune DTH response characterized by a selective increase of CD4⁺ IFN-γ producing cells and T cytotoxic cells, which result in the clearing of bacilli and concomitant tissue damage. In contrast, ENL reactions may be viewed as a transient dimunution of Ts cells and activity leading to a partial and transient augmentation in cell-mediated immunity, perhaps sufficient to result in antibody and immune complex formation, but insufficient to clear bacilli from lesions.