The Functional Activity of Fc Gamma RII and Fc Gamma RIII on Subsets of Human Lymphocytes

Subsets of human lymphocytes were isolated from peripheral blood using magnetic beads coated with anti-CD4, -CD8, -CD19 or -CD56 antibodies to yield T4, T8, B and natural killer (NK) cell suspensions with greater than 95% purity. The functional activity of Fc gamma receptor II (Fc gamma RII) and Fc gamma receptor III (Fc gamma RIII) on these subsets was assessed by measuring rosette formation with red cells sensitized with known levels of either rabbit IgG or human (monoclonal or polyclonal) IgG1 anti-D, IgG3 anti-D or IgG3 anti-c (E-IgG). Lysis of red cells by K cells (mediated by Fc gamma RIII) in antibody-dependent cell-mediated cytotoxicity (ADCC) assays was promoted by polyclonal and some monoclonal antibodies. Using these 'ADCC+' antibodies, minimum red cell sensitization levels required to promote rosette formation with NK cells were 2000 IgG1 or IgG3 molecules/red cell compared to 15,000 IgG1 or 4000 IgG3 molecules/red cell with 'ADCC-' monoclonal antibodies. The greater efficiency of ADCC+ antibodies is consistent with their previously reported ability to bind Fc gamma RIII via CH2 and CH3 domains whereas ADCC- antibodies bind only via CH3 domains. B cells formed rosettes only at high levels of sensitization: approximately 60,000 IgG1 or 20,000 IgG3 anti-D molecules/cell. These data reflect the low affinity of Fc gamma RII for monomeric human IgG. Although over 90% of NK cells bound anti-CD16, and 70% formed rosettes with red cells sensitized with rabbit IgG (30,000 molecules/cell), only 25% of NK cells formed rosettes with E-IgG3 at 100,000 IgG molecules/cell. Approximately 35% of B cells, 10% of T8 cells but no T4 cells formed rosettes with E-IgG (100,000 IgG3 molecules/cell). With T8, B and NK cells, IgG3 anti-D promoted greater rosette formation than IgG1 anti-D at comparable levels of sensitization. Presumably the longer hinge region of IgG3 enabled it to bridge the gap between negatively charged lymphocytes and red cells more efficiently than IgG1.