Monoclonal origin of B cells producing k, and k immunoglobulin light chains in a patient with chronic lymphocytic leukemia

Immunoglobulin gene rearrangements can be used as genetic markers of clonality in the study of B-cell populations [4]. We have therefore analysed the structure and expression of heavy and light chain immunoglobulin genes in lymphocytes of a patient with chronic lymphocytic leukemia, where we found both k and lambda producing B cells, but in most of the cells both k and lambda chains were co-expressed on the same surface membrane. Single rearrangements were observed in mu, JH, k and lambda DNA sequences, thus providing strong evidence for the monoclonal origin of the cells bearing different light chains. Moreover, the analysis of Ig sequence RNA showed, in addition to normal mu, k and lambda mRNA molecules, high levels of a small lambda related RNA sequence. These findings are discussed in relation to a model of B-lymphocyte differentiation which could be either an additional or an alternative hypothesis to the current one of isotypic exclusion.