Analysis of expressed VH genes in persistent polyclonal B cell lymphocytosis reveals absence of selection in CD27+IgM+IgD+ memory B cells

Abstract

Persistent polyclonal B cell lymphocytosis (PPBL) is a hematological disorder diagnosed predominantly in women, characterized by a polyclonal increase in the number of peripheral blood B lymphocytes. Abnormality of the B cell population was evidenced by the finding of multiple bcl-2/lg gene rearrangements and an additional long-arm chromosome within a significant proportion of B cells. To gain further insight about the developmental status of B lymphocytes in PPBL, analysis of cell surface lg receptors was undertaken. An important expansion of the CD27+lgM+lgD+ B cell population was noted in PPBL patients (n=4). When investigated by PCR, pattern of heavy chain variable region (VH) genes usage in patients (n=6) was shown to be similar to that observed in healthy individuals (n=3). In-depth investigation was then conducted through cloning and sequencing of individual VH genes in three of those patients. They were mostly found to be mutated (21/29), correlating with the observed increase in CD27 expression, a marker of memory B cells. Altogether, these data clearly point out to the exact nature of the expanding B cell subset in patients. Finally, analysis of the repartition of recombinant versus silent mutations in framework regions (FR) of lg genes showed no evidence of positive antigenic selection following somatic hypermutation. Thus, we suggest that a lack of response to physiological signals responsible for the elimination of low affinity memory lgM+lgD+ B cells in germinal centers could play an important role in the development of PPBL.