REACTIVE VERSUS NEOPLASTIC MONOCYTOID B-CELL PROLIFERATIONS - IN-SITUHYBRIDIZATION STUDY OF IMMUNOGLOBULIN LIGHT-CHAIN MESSENGER-RNA

To distinguish reactive versus neoplastic monocytoid B-cell (MBC) prol iferations, the clonality of MBC was examined in paraffin-embedded tis sues by in situ hybridization (ISH) of immunoglobulin (Ig) light chain messenger RNA (mRNA) with sensitive oligonucleotide probes in 26 case s. They included 13 cases of lymphadenitis with MBC reaction and 13 ca ses of nodal (n = 8) and extranodal (n = 5) monocytoid B-cell lymphoma (MBCL). Two cases represented a composite lymphoma showing a centrobl astic-centrocytic and MBCL component. The clonality of MBC infiltrates could be demonstrated in 16 of 26 (61.5%) cases by immunostaining for Ig light chains and in all(100%) cases by ISH. Neoplastic MBC usually expressed a faint-to-moderate light chain restriction of mRNA, wherea s some MBC (10% to 30% of total MBCL population) showed a strong posit ivity irrespective of plasmacytoid differentiation as indicated by Ig immunostaining (present in 9 of 13 cases). Reactive MBC expressed a fa int kappa and lambda light-chain mRNA positivity. Five percent to 20% of total reactive MBC showed also a strong positivity for both Ig ligh t chain mRNA, although only a minor part of these cells (7 of 13 cases ) expressed polyclonal Ig by immunohistochemistry. These results indic ate that (1) bath reactive and neoplastic MBC can differentiate into p lasma cells; and (2) a relatively high percentage of reactive and neop lastic MBC show a detectable mRNA transcription, but not a correspondi ng Ig synthesis. Either the Ig detection is not sensitive enough or th ese cells might be in an early differentiation phase, where the Ig pro duction has not yet started.