In situ analysis of the variable heavy chain gene of an IgM/IgG-expressing follicular lymphoma - Evidence for interfollicular trafficking of tumor cells

is generally assumed that follicular lymphomas (FL) not only morphologically resemble normal germinal centers but have retained some functional characteristics of their non-neoplastic counterparts as well. Recent IgV gene analyses on a panel of FLs however, strongly suggested that FLs do not retain the capacity of somatic hypermutation and are not being selected on basis of the quality of their mIgV regions. To extend these findings, we investigated the follicular organization and class switching in a FL that consisted of both IgM- and IgG-expressing tumor cells with a high somatic mutation load and significant intraclonal V-H gene diversity. V-H-Cmu and V-H-Cgamma gene transcripts were amplified and sequenced from samples of approximately 50 tumor cells, isolated from frozen tissue sections by laser microdissection. We identified many different subclones and obtained limited evidence of subclone dominance in individual follicles. Remarkably, several subclones were found scattered over different follicles. All samples contained IgM- and IgG-expressing tumor cells with, in general, nonidentical mutation patterns, which is not in support of ongoing class switching. Accordingly, no switch circle recombination products were found. The findings indicate that the neoplastic follicles lack the organization and functions typical of reactive germinal centers