Somatic hypermutation unlocks antibody specificities beyond the primary repertoire.

B cell somatic hypermutation (SHM) and selection in germinal centers (GCs) enhance antibody affinity for antigen. Here, we investigated whether SHM-based antibody evolution is restricted to specificities established through V(D)J recombination in the primary repertoire. Tracking pre-defined non-specific B cells across multiple immunization models revealed that non-cognate B cells within GCs undergo SHM. Under conditions of limited B cell competition, these B cells generated de novo antigen recognition to multiple epitopes across diverse model antigens. Phylogenetic analyses identified diverse mutational pathways leading to new antigen affinities, and enhanced T cell co-stimulation further promoted new antigen recognition. Our data support a model in which B cell competition-rather than an intrinsic requirement for specific affinity-limits the emergence of new affinities through SHM, highlighting the mammalian adaptive immune system's ability to explore antibody-antigen interactions beyond those encoded by the V(D)J-dependent primary repertoire, demonstrating the flexibility of SHM in not only ripening but also reshaping specificity.