IgG + memory B cells were isolated from the selected donors and immortalized with EBV in the presence of CpG as described [41] . The immortalization efficiency of memory B cells from HIV-1 patients was significantly lower than that of non-HIV-1-infected donors (3% versus 20%, n = 21, p,0.001). This finding may be explained by the recent report that HIV-specific B cells are present within a population of ''exhausted'' memory B cells, characterized by the expression of inhibitory receptors and low levels of CD21 [56, 57] . To maximize detection of antibodies recognizing conserved features, cell culture supernatants were screened by parallel highthroughput ELISAs using five recombinant HIV-1 Env proteins, including trimeric gp140 [48] , monomeric gp120 and gp41 recombinant ectodomains. From the 21 donors interrogated we selected 58 B cell clones producing mAbs that bound to at least one of the screening antigens. The mAbs were purified and further characterized for binding specificity and neutralizing activity using an extended panel of recombinant Env proteins and pseudoviruses representative of several HIV-1 clades with diverse coreceptor usage, geographic origin and conformation.