Studies for vaccine and human therapeutic Ab development in cynomolgus monkeys (cynos) are influenced by immune responses, with Ab responses playing a significant role in efficacy and immunogenicity. Understanding the nature of cyno humoral immune responses and characterizing the predominant cyno IgG types produced and the Fc-FcγR interactions could provide insight into the immunomodulatory effects of vaccines. Anti-drug Ab responses against human IgG therapeutic candidates in cynos may affect efficacy and safety assessments because of the formation of immune complexes. There is, however, limited information on the structure and function of cyno IgG subclasses and how they compare with human IgG subclasses in Fc-dependent effector functions. To analyze the functional nature of cyno IgG subclasses, we cloned four cyno IgG C regions by using their sequence similarity to other primate IgGs. The four clones, cyno (cy)IGG1, cyIGG2, cyIGG3, cyIGG4, were then used to construct chimeric Abs. The sequence features of cyno IgG subclasses were compared with those of rhesus monkey and human IgG. Our data show that rhesus monkey and cyno IgG C regions are generally highly conserved, with differences in the hinge and hinge-proximal CH2 regions. Fc-dependent effector functions of cyno IgG subclasses were assessed in vitro with a variety of binding and functional assays. Our findings demonstrate distinctive functional properties of cyno IgG subclasses. It is notable that human IgG1 was less potent than cyno IgG1 in cyno FcγR binding and effector functions, with the differences emphasizing the need to carefully interpret preclinical data obtained with human IgG1 therapeutics.
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