Early B blasts acquire a capacity for Ig class switch recombination that is lost as they become plasmablasts.

Rapid production of neutralizing antibody can be critical for limiting the spread of infection. Such early antibody results when B blasts mature directly to plasmablasts without forming germinal centres. These extrafollicular responses can involve Ig class switch recombination (CSR), producing antibody that can readily disseminate through infected tissues. The present study identifies the differentiation stage where CSR occurs in an extrafollicular response induced by 4-hydroxy-3-nitrophenyl acetyl (NP) conjugated to Ficoll (NP-Ficoll). To do this we took advantage of the antigen dose dependency of CSR in this response. Thus, while both 30 mg and 1 mg NP-Ficoll induces plasmablasts, only the higher antigen dose induces CSR. Activation-induced cytidine deaminase (AID) is critical for CSR and in keeping with this a proportion of NP-specific B blasts induced by 30 mg NP-Ficoll express AID. None of the B blasts responding to the non-CSR-inducing 1 mg dose of NP-Ficoll express AID. We confirmed that CSR occurs in B blasts by demonstrating the presence of rearranged heavy chain transcripts in B blasts in the 30 mg response. CSR in this extrafollicular response is confined to B blasts, because NP-specific plasmablasts, identified by expressing CD138 and Blimp1, no longer express AID and cannot undergo CSR.