Vaccine studies conducted in both non human primate models A

Vaccine studies conducted in both non-human primate models (Alpert et al., 2012; Gómez-Román et al., 2005; DeVico et al., 2007; Hidajat et al., 2009; Xiao et al., 2010; Brocca-Cofano et al., 2011; Barouch et al., 2012) and humans (Forthal et al., 2007; Haynes et al., 2012) have also suggested that ADCC activity may provide some protection against infection. Early evidence from the Vax004 Phase III efficacy study identified an inverse correlation between ADCVI activity and infection (Forthal et al., 2007). Most recently, an immune-correlates analysis of the RV144 vaccine trial showed a correlation between higher ADCC activity and protection from infection among vaccinees with low plasma IgA, lending additional support for the potential of ADCC-mediating c-src to protect (Haynes et al., 2012; Tomaras et al., 2013). ADCC-mediating antibodies target a wide range of epitopes on the envelope (Env) protein, as well as epitopes on Gag (Grunow et al., 1995), Pol (Isitman et al., 2012), Nef (Yamada et al., 2004) and Vpu (Wren et al., 2013; Tiemessen et al., 2009) proteins, though the significance of ADCC activity directed against proteins other than Env remains unclear. Within Env, ADCC-mediating antibodies target epitopes similar to neutralizing antibodies, including the membrane proximal external region (MPER), CD4 binding site (CD4bs) and V1-V2 and V3 regions as well as unique epitopes on the gp120 inner domain naturally occluded by gp41 and exposed following CD4 binding (CD4-induced [CD4i] epitopes) (reviewed in (Pollara et al., 2013; Gach et al., 2011)). A detailed study by Guan et al. (2013) finely mapped three unique clusters (A, B and C) of CD4i-specific antibodies using a competition-based ELISA approach (Guan et al., 2013). Cluster A epitopes become exposed following the conformational change resulting from Env binding to cellular CD4 that occurs during viral entry or following infection and subsequent cell–cell spread when CD4 and the viral envelope are co-expressed on the surface of the same cell (Finnegan et al., 2001; Acharya et al., 2014; Veillette et al., 2014). Prototypic Cluster A mAbs A32 and C11 bind to similar epitopes on gp120 but only weakly cross-compete with each other, suggesting they have unique specificities (Guan et al., 2013). Fab inhibition experiments have demonstrated that A32-like antibodies frequently constitute the majority of the ADCC activity observed in chronically-infected (Ferrari et al., 2011) and RV144-vaccinated (Bonsignori et al., 2012) individuals. The Cluster B epitope is defined by only one antibody, N12-i15, and targets a conformational epitope on gp120 that requires CD4 binding and involves the V1–V2 loop. Cluster C antibodies recognizing the co-receptor binding sites (CoRBS) are the only CD4i antibodies capable of neutralizing cell-free virus and are further sub-categorized into 4 groups as specified by competition with either 17b or 19e mAbs. Among these ADCC-mediating antibodies, the A32 and C11 mAbs from cluster A are the most potent (Guan et al., 2013), and these CD4i mAbs have non-overlapping specificities. Few studies have examined ADCC mAb breadth or the ability to recognize diverse envelopes from different HIV-1 clades. Recently, two studies identified epitope breadth (Wren et al., 2013) and increased cross-clade ADCC activity (Madhavi et al., 2014) as characteristics associated with superior ADCC-mediating antibody responses. Despite this work, we still know very little about the features defining a protective ADCC-mediating immune response or the optimal methods required to dissect this response at the monoclonal level. Further, few studies have attempted to define the contribution of epitope-specific mAbs other than A32 to the overall ADCC response or to determine the kinetics of ADCC Ab development following infection. While considerable progress in identifying HIV-specific bnAbs has been made using high-throughput methods such as protein-specific screens and microneutralization assays, no high-throughput, functional methods for isolating ADCC-mediating antibodies have been described.