Pseudoplaque formation by cytophilic immunoglobulins in the sera of lupus patients as measured using reverse haemolytic plaque assay.

Normal human peripheral blood lymphocytes cultured in vitro with lupus sera (LS) were studied for their possible immunoglobulin (Ig) secretion using reverse haemolytic plaque assay (RHPA). Plaque forming cells (PFC) produced in cultures with LS (2,323 +/- 858 PFC/10(6) cultured cells) significantly increased in number compared to the control cultures with normal human serum (206 +/- 43, P less than 0.05). However, the artifactual nature of the PFC formation due to cytophilic Ig in LS subsequently became evident from the kinetic curves, trypsinization or medium change at the end of the culture period, radiation of cells on the first day of culture, or by depletion of nylon wool column-adherent cells from the culture system. These cytophilic Ig were observed in 56% of the sera obtained from untreated lupus patients. Thus, their possible interference with the resultant spurious plaque formation might be difficult to eliminate when performing RHPA in lupus patients.