Neoantigen of the membrane attack complex of human complement: Occurrence on peripheral blood leukocytes from patients with systemic lupus erythematosus.

Since complement activation and hematological abnormalities occur in systemic lupus erythematosus (SLE), the present study is an investigation of whether the membrane attack complex of complement might be bound to peripheral blood leukocytes (PBL) in vivo. Assembly of the membrane attack complex results in the generation of neoantigen (neoAg) which is complex-specific and not expressed by any of the individual complement proteins. FITC antiserum specific to neoAg was employed to detect the membrane attack complex on PBL from 7 normal donors, 12 patients with SLE, and 2 patients with rheumatoid arthritis (RA): 3 +/- 1% of normal, 25 +/- 13% of SLE, and 23 +/- 11% of RA PBL were positive. The majority of the neoAg positive PBL in SLE were polymorphonuclear neutrophils (PMN) as shown by adherence to plastic, phagocytosis of carbonyl iron, and differential cell counts. The PBL were greater than 98% viable as indicated by the trypan blue exclusion technique. These observations strongly suggest that the membrane attack complex may be bound to viable PBL in patients with SLE and RA, and further raise the possibility that the membrane attack complex, may have a function other than lysis.