Histogenesis of CD5-positive and CD5-negative B-cell neoplasms on the aspect of somatic mutation of immunoglobulin heavy chain gene variable region.

The immunoglobulin heavy chain (IgH) gene of B-cells dramatically alters twice in their differentiation to memory or plasma cells; VDJ recombination at B-cell precursor and somatic hypermutation, class switch recombination and receptor revision at germinal center (GC) B-cells. Among them, somatic hypermutation of the IgH gene variable region (VH gene) is a powerful tool for detection of B cell differentiation. B-cells and B-cell neoplasms have been divided into following; 1) pre-GC B-cells and neoplasms with a germline VH gene and 2) GC and post-GC B-cells and neoplasms with a somatically mutated VH gene. In this article, we review normal B-cell differentiation and histogenesis of various types of B-cell neoplasms on the aspect of somatic mutation of the rearranged VH gene. In particular, differences between CD5+ and CD5- B-cell neoplasms, using our own data of over 100 cases with B-cell neoplasms, are discussed. Although CD5+ B-cells are included in pre-GC B-cells for the reason of germline VH gene in most of CD5+ B-cells, an about 5% of CD5+ B-cells show somatically mutated VH gene. The rearranged VH gene of CD5+ B-cell neoplasms shows heterogeneity, whereas CD5- B-cell neoplasms possess somatically mutated VH gene with a mean of 8 approximately 12%. Both CD5+ B-cell chronic lymphocytic leukemia and CD5+ diffuse large B-cell lymphoma display that about half of cases show a germline or low frequency of somatic mutation and the others possess somatically mutated VH gene. CD5+ mantle cell lymphoma constitutes most cases with germline and a small number of cases with mutated VH gene. Therefore, CD5- B-cells & CD5- B-cell neoplasms are distinct from CD5+ B-cells and CD5+ B-cell neoplasms in somatic mutation of VH gene. It suggests that each of CD5- and CD5+ B-cells independently has its own differentiation.