The first dose of a Haemophilus influenzae type b conjugate vaccine reactivates memory B cells: Evidence for extensive clonal selection, intraclonal affinity maturation, and multiple isotype switches to IgA2

The Ab response of a healthy adult to the first dose of a Haemophilus influ enzae type b capsular polysaccharide (HibCP) conjugate vaccine was studied at the level of Ig gene usage by circulating Ah-secreting cells. Forty-one IgA and 17 IgG mRNA sequences were obtained. The major part of the response was confined to IgA Ab-secreting cells, and 72% of the IgA sequences were derived from the progeny of a single rearranged B cell. These sequences cou ld be arranged in a genealogical tree showing multiple somatic mutations an d at least two intraclonal isotype switches to IgA2, Fourteen somatic mutat ions were shared by this clonal progeny, indicating that extreme clonal sel ection had occurred early in the clonal development. Taking into account th e frequency of somatic mutations and the clone size, it was evident that th e responding cell population must have originated from a mutated, highly se lected, and expanded population of cells existing before vaccination, i.e., memory B cells, The dominating heavy and light chains of the response were combined in a Fab that bound HibCP. It was shown that the shared heavy and light chain mutations increased the affinity for HibCP considerably, indic ating that the clonal selection had been driven by affinity. Pre-existing m emory cells in unvaccinated adults may explain several features of Ab respo nses to polysaccharide vaccines and may play a role in acquiring the abilit y to respond to pure polysaccharides during infancy.