Erythema nodosum leprosum is associated with up-regulation of polyclonal IgG1 antibody synthesis.

Erythema nodosum leprosum (ENL) is a serious complication of lepromatous (L) disease in leprosy. We have previously shown that of the four IgG subclasses, IgG1 and IgG3 Mycobacterium leprae-specific antibodies are significantly lower in leprosy patients during ENL reaction compared with untreated L patients. To see if this decrease results from a down-regulation of antibody synthesis during ENL, the frequency of antibody-secreting B cells (ABSC) in the blood compartment was determined by ELISPOT and related to serum immunoglobulin concentrations (microgram/ABSC). Control groups consisted of 16 patients with stable L disease and 32 healthy endemic controls (EC). Paired samples were analysed during acute ENLS (n = 13) and after the reaction had subsided to identify changes associated with ENL. Polyclonal (PC) IgG1 was elevated in L patients compared with EC (325 micrograms versus 180 micrograms). Interestingly, patients during acute ENL showed concentrations higher than L patients (419 micrograms), which decreased after the reaction had subsided (260 micrograms), indicating the transient nature of the antibody response. IgG2 antibodies showed the reverse trend and were lower during ENL and increased after the reaction had subsided. The mean concentrations for PC IgG3 and IgG4 antibodies were similar during ENL and after the reaction had subsided. Thus, decrease in M. leprae-specific IgG1 and IgG3 antibodies is not related to down-regulation of B cell responses. Identification of factors which regulate PC IgG1 antibody synthesis may provide additional insights into determinants of ENL reactions.