EXISTENCE OF A SOLUBLE FORM OF CD50 (INTERCELLULAR-ADHESION MOLECULE-3) PRODUCED UPON HUMAN LYMPHOCYTE-ACTIVATION - PRESENT IN NORMAL HUMANSERUM AND LEVELS ARE INCREASED IN THE SERUM OF SYSTEMIC LUPUS-ERYTHEMATOSUS PATIENTS

CD50 (ICAM3) is a leukocyte differentiation Ag expressed almost exclus ively on hemopoietic cells, with a key role in the first steps of immu ne response. To develop a specific sandwich ELISA to detect a soluble CD50 form (sCD50), two different mAbs (140-11 and 101-1D2) recognizing non-overlapping epitopes were used. sCD50 was detected in the superna tant of stimulated PBMCs, with the highest levels after CD3 triggering . Simultaneously, the CD50 surface expression diminished during the fi rst 24 h. sCD50 isolated from culture supernatant and analyzed by immu noblotting showed an apparent m.w. of 95 kDa, slightly smaller than th e membrane form. These data, together with Northern blot kinetics anal ysis, suggest that sCD50 is cleaved from cell membrane. Furthermore, w e detect sCD50 in normal human sera and higher levels in sera of syste mic lupus erythematosus (SLE) patients, especially in those in active phase. The sCD50 levels showed a positive correlation with sCD27 level s (r = 0.4213; p = 0.0026). Detection of sCD50, both after in vitro CD 3 triggering of PBMCs and increased in SLE sera, suggests that sCD50 c ould be used as a marker of lymphocyte stimulation.