Structural analysis of immunodominance in the antibody response to influenza virus haemagglutinin

The protective immune response to influenza virus is directed against the surface glycoprotein haemagglutinin (HA) which undergoes continuous antigenic drift in order to evade immune recognition. The recognition sites for neutralising antibodies have been located to five surface accessible regions (sites A-E) on the membrane distal ectodomain of the HA1 subunit. Although all five sites are recognised by neutralising antibodies, there is evidence for haplotype specific differences and immunodominance in the response to natural infection. BALB/c (H-2d) Mab predominantly recognise HA1 198, and CBA/Ca (H-2h) Mab, HA1 158. Restriction of the immune response and the observed immunodominance raises several important questions concerning the mechanisms of repertoire selection, which this investigation aims to address using BALB/k (MHC congenic) mice, to evaluate possible effects of MHC background on selected repertoires. The BALB/k antibody response to X31 HA was found to be a composite of both repertoires, recognising predominantly HA1 198(Ala→Glu) in four out of five donors and HA1 158(Gly→Glu) in the fifth, suggesting both non-MHC and MHC effects. Previous studies on antibody gene usage have shown restriction for anti-haptenic responses and diversity for anti-protein responses. Generally, antibody specificity for proteins is diverse. Since I had found immunodominance in the response to influenza HA protein, it was possible that antibody gene usage would resemble a hapten-like response and thereby account for this highly focused response. Extensive sequence analysis of antibody gene usage by influenza HA specific Mab revealed a diverse VL usage but restricted VH family usage by BALB/k HA1 198 specific Mab and very restricted VL and donor dependent restricted VH family usage by CBA/Ca HA1 158 specific Mab. VL and VH family usage was also mutually exclusive between the two strains and specificities. BALB/k HA1 158 specific Mab (a CBA/Ca-like response) were a composite of both repertoires, with very restricted VH family usage, but with diverse VL usage. It was evident that the immunodominant response per individual was a consequence of clonal expansion of one to three progenitor cells from both BALB/k (recognition of HA1 198), and CBA/Ca (recognition of HA1 158), however antibody gene usage appears to be stochastic in the BALB/k HA1 158 specific response. Furthermore, the observation that the same VL/VH family combinatorial association by Mab of two different specificities from two donors and such diverse VH/VL gene family usage indicates that fine specificity and immunodominance cannot be attributed to antibody V gene usage alone