Stage and isotype specific immune responses in a rat model of filariasis

Inbred PVG rats infected with 100 Brugia pahangi infective larvae (L3) divide into rats that develop microfilaraemic infection (Mf+), and those that remain microfilaria negative (Mf-). All rats had high levels of specific IgG to adult worm and L3 extracts, however, after the onset of microfilaraemia, Mf+ rats had significantly higher levels of IgG to Mf extract. Mf+ rats recognized several antigenic components of each developmental extract that were not responded to by Mf- rats. In particular Mf+ rats recognized a triplet of proteins of 61-67 kD in microfilarial extract from day 74 post infection onwards. This indicated that these proteins were stage specific to Mf and Mf- rats had not been exposed to Mf rather than Mf absence being due to a protective antibody response. Analysis of the immunoglobulin isotype usage during infection revealed that each isotype was independent both in its period of induction and the developmental stage to which it responded. The predominant isotypes responding throughout infection were IgG1, IgG2a and IgM. Specific IgG2b and IgG2c were elevated early in infection but after the onset of microfilaraemia antibody of these subclasses was suppressed. The antigenic profiles recognized on immunoblots by IgG1, IgG2a and IgM were very similar.