Evidence for an antigen-driven humoral immune response in medullary ductalbreast cancer

A minority of breast cancers is characterized by lymphoplasmacytic. infiltr ates that have been correlated with improved patient survivals. The associa tion of improved prognosis with plasmacytic. infiltrates has been classical ly linked with the rare medullary carcinoma subtype but is also evident in the smaller infiltrated fraction of the more abundant nonmedullary (not oth erwise specified) tumors. It is our hypothesis that these plasma cell (PC) infiltrates represent a host humoral response driven by one or more tumor-d erived neoantigens. As the index study group, two primary medullary carcino ma tumors were examined. Immunophenotyping confirmed a large number of IgG PCs in contradistinction to normal breast, which typically contains a lesse r number of mainly IgA isotypes. IgG heavy and light chains were expressed as combinatorial phage Fab libraries. VH and VL sequences showed a preponde rance of clonal groups in both patients, as identified by germ-line gene us age and junctional mutation patterns. Panning of phage Fab libraries agains t purified antigens excluded Her2/neu and p53 as the eliciting antigen, and failure of clonal enrichment by cell panning suggested that the neoantigen was not membrane expressed or was expressed at low levels. Cognate, origin al VH+VL pairs were obtained by single cell PCR of tumor PCs, which showed overlap with the pooled IgG libraries. Tumor-derived IgG V genes exhibited mutational patterns that were consistent with antigenic selection and affin ity maturation. Where examined, IgG1 was the predominant isotype, consisten t with a T-dependent (i.e., protein) antigen. From these data, we infer tha t the breast tumor PC infiltrates of the medullary carcinoma subtype are co mpatible with an autogenic. tumor neoantigen-driven humoral immune response .