Cellular origin and clonality of classic Hodgkin's lymphoma: immunophenotypic and molecular studies.

The cellular origin of Hodgkin and Reed-Sternberg (HRS) cells, the neoplastic cells of classic Hodgkin's lymphoma (HL), resisted clarification until the second half of this decade. One major obstacle to successful experimental investigations was the rarity of the HRS cells in the tissue affected by HL. Immunophenotypical studies using monoclonal antibodies already pointed in the early 1980s towards a lymphocytic origin for HRS cells, but were not definitive because of the usually variable expression of B-cell and/or T-cell antigens, and the additional expression of markers typical for other cell lineages, especially dendritic cells. Attempts to elucidate the cellular derivation of HRS cells by demonstrating the clonal rearrangements of the immunoglobulin (Ig) or T-cell receptor (TCR) genes in the DNA of whole-tissue extracts also remained inconclusive due to inconsistent results. In frustration with whole-tissue DNA studies, genetic approaches were turned to the single cell level. Among the techniques developed for the isolation of HRS cells, the extraction of single CD30+ HRS cells from immunostained frozen sections by means of hydraulic micromanipulation proved to be the most suitable. Using this method, monoclonal Ig gene rearrangements were detected in the HRS cells in 36 of 38 (95%) HL cases. Sequence analysis demonstrated high loads of somatic mutation in the rearranged variable regions. Molecular investigation of three cases of HL occurring in association with non-Hodgkin's lymphomas (NHLs) showed that all of the lymphoma lesions had an identical precursor with the molecular features of a germinal center B cell. In summary, these findings indicate that (1) approximately 95% of classic HLs originate from B cells; (2) the direct cellular precursors of the HRS cells are germinal center B cells; (3) the transforming event that causes HL leads to the complete morphologic and immunophenotypical change of the HRS cell precursors; and (4) the HRS cell population of a given case exclusively arises from a single transformed cell and expands by clonal growth. It remains to be shown whether the 5% of HLs for which a B-cell derivation could not be demonstrated are T-cell related.