Do myelomatous plasma cells really express surface immunoglobulins?

Surface immunoglobulins (sIg) are traditionally considered to be absent in plasma cells (PC). However, it has recently been reported that up to one third of myeloma patients are positive for surface immunoglobulins. Nevertheless, this observation has not been confirmed in other studies. In order to assess whether or not sIg are really expressed by myelomatous PC and to exclude possible staining of either cytophilic or cytoplasmic Igs, simultaneous experiments were carried out with i) incubation at 37 degrees C, ii) blocking with non-conjugated anti-Ig light chains, and iii) cytoplasmic staining after cell membrane fixation and permeabilization. Triple staining for CD38/kappa/lambda was used in all cases and the staining intensity was quantitated in MESF (molecule equivalents of soluble fluorochrome). In addition, 20 B-CLL cases and 10 healthy donors were used as reference controls. Our study shows that 7 out the 20 patients (35%) analyzed expressed sIg and that the surface staining was specific.