Formation of antibody to matrix protein in experimental human influenza A virus infections

Antibodies to type A influenza virus matrix protein (M) were assayed by single radial diffusion in 180 paired sera of volunteers challenged intranasally with live H3N2 viruses of varying degrees of virulence. Of these volunteers 20 had had severe clinical reactions (influenza-like); there had been 19 moderate reactions (lesser degrees of constitutional illness), and the remaining 141 reactions had been graded mild, very mild, or nil. Only 2 volunteers were shown to have antibodies to M in the pre-trial serum samples, and 11 developed anti-M rises after virus inoculation. Nine of the 11 had had severe reactions, and 2 had had moderate reactions. There was, therefore, a clear correlation between severity of clinical illness and anti-M antibody formation. In general, anti-M increases coincided with increases to the hemagglutinins and nucleoprotein, and with virus shedding. However, no anti-M antibody could be demonstrated in paired sera of 18 additional volunteers of whom 12 had developed severe reactions after the inoculation of virulent H0N1 and H1N1 influenza A viruses and of whom 12 had shown laboratory evidence of infection.