The mechanism of rosette formation between Rh (D)-positive erythrocytes and peripheral blood lymphocytes from Rh isoimmunized individuals. The role of surface micro-projections.

Peripheral blood leucocytes were obtained from Group A Rh (D)-negative male volunteers re-injected with A Rh (D)-positive erythrocytes and the mechanism by which they form rosettes with A Rh (D)-positive erythrocytes was studied. It was found that there was a marked reduction in the number of rosettes formed at 4° as compared with 37°. Moreover, those rosettes formed at 4° comprised a central rosette-forming cell surrounded by one layer of erythrocytes whereas at 37° some rosette-forming cells (RFC) appeared to be surrounded by agglutinated erythrocytes. No anti-D antibody-forming cells were detected by the localized haemolysis in gel technique despite the presence of large numbers of RFC in the preparations tested. Pretreatment of leucocyte aliquots with anti-sera specific for human Fab at 4° was nearly as effective as pretreatment with anti-Fab at 37° in inhibiting rosette formation. Examination of serial sections of rosettes with the electron microscope revealed that erythrocytes were often attached to micro-projections from the surface of central RFC. Cytochalasin B partially inhibited rosette formation at 37°. Those rosettes formed in the presence of cytochalasin B consisted of a central rosette-forming cell with few adherent erythrocytes and with the electron microscope it was seen that erythrocytes were rarely attached to central RFC by micro-projections. It is suggested that in this system the contractile activity of the surface of rosette-forming cells is important in rosette formation.