Identification of IgM as the leprosy patient serum factor responsible for rapid sedimentation of formolized sheep erythrocytes.

The serum of some leprosy patients with impaired specific cellular immunity for Mycobacterium leprae causes rapid sedimentation of formolized sheep erythrocytes, a phenomenon known as the Rubino reaction. The Rubino factor was precipitated from positive sera by 5% (w/v) polyethylene glycol (PEG), bound to a concanavalin A (ConA)-Sepharose column and eluted with D-mannose, and was also eluted from a Mono Q column, pH 8.0, with 0.4 M NaCl. The Rubino factor was eluted in a volume which coincided with that of human serum IgM from a Sepharose 6 column. IgM was present in the preparation obtained by this sequence of chromatographic procedures. The correspondence of IgM with the Rubino factor was demonstrated by the following data: a) the Rubino factor was adsorbed to rabbit IgG antihuman IgM-agarose and the activity was recovered in the acid eluate of the column; b) the Rubino reaction was inhibited in the presence of rabbit antihuman IgM antibodies. This behavior was not observed when the same procedures were carried out using anti-alpha 2-macroglobulin antibodies as a control. The rapid sedimentation of formolized sheep red cells caused by the serum of lepromatous leprosy patients was not inhibited by phenolic glycolipid-I, suggesting that the IgM responsible for the Rubino reaction is not directed to this antigen which is specific for M. leprae. There was no correlation between the positivity of the Rubino reaction and the increase in total serum IgM levels observed in 42% of the lepromatous patients evaluated. The demonstration that the Rubino factor is an IgM now permits the identification of the epitope recognized by it, and this may be used as a tool to understand the specific cellular immune unresponsiveness which characterizes lepromatous leprosy.