HIV-1 gp41 envelope antibodies, which are frequently induced in HIV-1-infected individuals, are predominantly nonneutralizing. The rare and difficult-to-induce neutralizing antibodies (2F5 and 4E10) that target gp41 membrane-proximal epitopes (MPER) are polyspecific and require lipid binding for HIV-1 neutralization. These results raise the questions of how prevalent polyreactivity is among gp41 antibodies and how the binding properties of gp41-nonneutralizing antibodies differ from those of antibodies that are broadly neutralizing. In this study, we have characterized a panel of human gp41 antibodies with binding specificities within the immunodominant cluster I (gp41 amino acids [aa] 579 to 613) or cluster II (gp41 aa 644 to 667) for reactivity to autoantigens, to the gp140 protein, and with MPER peptide-lipid conjugates. We report that while none of the gp41 cluster I antibodies studied were polyspecific, all three gp41 cluster II antibodies bound either to lipids or autoantigens, thus showing the propensity of cluster II antibodies to manifest polyreactivity. All cluster II gp41 monoclonal antibodies (MAbs), including those that were lipid reactive, failed to bind to gp41 MPER peptide-lipid complexes. Cluster II antibodies bound strongly with nanomolar binding affinity (dissociation constant [K d ]) to oligomeric gp140 proteins, and thus, they recognize conformational epitopes on gp41 that are distinct from those of neutralizing gp41 antibodies. These results demonstrate that lipid-reactive gp41 cluster II antibodies are nonneutralizing due to their inability to bind to the relevant neutralizing epitopes on gp41.Anti-HIV-1 gp41 envelope (Env) antibodies (Abs) are frequently induced in HIV-1-infected individuals (6,8,58). The antigenic determinants of gp41 have been mapped with a large panel of human gp41 antibodies (21,32,55,61). Whereas cluster I monoclonal Abs (MAbs) are directed against the immunodominant region (gp41 amino acids [aa] 579 to 613) of the gp41 envelope, a subset of human HIV-1 MAbs specific for "cluster II" (55) show binding specificities for the gp41 region, which includes HR-2 (heptad repeat 2) (aa 644 to 667) (21, 35, 55). The broadly neutralizing MAbs 2F5 and 4E10 recognize epitopes that are within the gp41 membrane-proximal external region (MPER), with the 2F5 epitope being adjacent to the cluster II region and that of 4E10 mapping outside the cluster II and being more membrane proximal (cluster III). There is some overlap between cluster II epitopes and the epitope recognized by MPER MAb 2F5 (56). Thus, cluster II human can partially cross-block 2F5 MAb binding to HIV-1 Env gp140 oligomers (3).Anti-gp41 antibodies whose epitopes are within the gp41 HR-2 region are largely nonneutralizing, with only rare neutralizing MAbs (9,36,45,61). One each of cluster I (clone 3) and cluster II (98-6) MAbs have been reported to weakly neutralize select HIV-1 strains (19,25). Both cluster I and cluster II MAbs, however, can bind HIV-1-infected cells and HIV-1 virions and also have been reported to ...