Neither did we find evidence indicating that the rabbits developed anti-coreceptor binding site antibodies: there was no enhancement of neutralization of HIV-2 by the immune rabbit sera in the presence of sCD4. There are two major groups of broadly cross-neutralizing mAbs that recognize different conformational,Gefitinib glycan-dependent epitopes. These groups are represented in our study by the mAbs PGT121 and PGT126, on the one hand, and PG9 and PG16, on the other, and are distinguished by the specific glycans upon which they depend for epitope recognition. Not only was the presence of either of these glycans not required for neutralization by our immune rabbit sera, the neutralizing effects of the sera were dramatically increased when both of these glycans were absent. Antibodies against the membrane proximal external region of gp41 may exhibit broadly cross-reactive neutralization. We did not test the rabbit sera for antibodies with that specificity, but consider it very unlikely that such antibodies were induced by the gp140 used here, since they have not been induced by similar antigens in other studies. The rabbits did develop vigorous responses against the V1/V2 region represented in the gp70-V1V2 peptide we tested in ELISA. Antibodies against variable regions of the Env could contribute to the neutralizing activity in our rabbit sera, but direct evidence of such was not obtained in this study. A major and enduring challenge in HIV vaccine development is induction of neutralizing antibody responses that are both potent and durable. To date,GSK1120212 neutralizing responses that have been induced in animal models have been of limited potency and non-durable, raising questions about the extent to which B cells that produce immunoglobulin molecules that neutralize are actually induced by the immunogens. The antibodies we induced in this study did exhibit cross-reactive neutralization, but the potency of the responses was limited. To evaluate whether the B cell responses of the rabbits were weakly induced we studied frequencies of B cells or plasma cells in spleens producing antibodies that bound soluble R2 gp140-GCN4-L trimer.