Research Article: Prime-Boost Immunization of Rabbits with HIV-1 gp120 Elicits Potent Neutralization Activity against a Primary Viral Isolate

Date Published: January 9, 2013

Publisher: Public Library of Science

Author(s): Kristin M. Narayan, Nitish Agrawal, Sean X. Du, Janelle E. Muranaka, Katherine Bauer, Daniel P. Leaman, Pham Phung, Kay Limoli, Helen Chen, Rebecca I. Boenig, Terri Wrin, Michael B. Zwick, Robert G. Whalen, Welkin E. Johnson.


Development of a vaccine for HIV-1 requires a detailed understanding of the neutralizing antibody responses that can be experimentally elicited to difficult-to-neutralize primary isolates. Rabbits were immunized with the gp120 subunit of HIV-1 JR-CSF envelope (Env) using a DNA-prime protein-boost regimen. We analyzed five sera that showed potent autologous neutralizing activity (IC50s at ∼103 to 104 serum dilution) against pseudoviruses containing Env from the primary isolate JR-CSF but not from the related isolate JR-FL. Pseudoviruses were created by exchanging each variable and constant domain of JR-CSF gp120 with that of JR-FL or with mutations in putative N-glycosylation sites. The sera contained different neutralizing activities dependent on C3 and V5, C3 and V4, or V4 regions located on the glycan-rich outer domain of gp120. All sera showed enhanced neutralizing activity toward an Env variant that lacked a glycosylation site in V4. The JR-CSF gp120 epitopes recognized by the sera are generally distinct from those of several well characterized mAbs (targeting conserved sites on Env) or other type-specific responses (targeting V1, V2, or V3 variable regions). The activity of one serum requires specific glycans that are also important for 2G12 neutralization and this serum blocked the binding of 2G12 to gp120. Our findings show that different fine specificities can achieve potent neutralization of HIV-1, yet this strong activity does not result in improved breadth.

Partial Text

A major challenge in developing a protective vaccine for HIV-1 is the identification of an immunogen that can elicit potent and broad-spectrum neutralizing antibodies to primary isolates [1], [2]. Efforts to identify and characterize monoclonal antibodies (mAbs) from humans have provided important insights into the targets and molecular mechanisms of HIV-1 neutralization [3]–[13]. However, using this knowledge to rationally develop an effective vaccine continues to be difficult [14], thus highlighting the need for empirical approaches in HIV-1 vaccine research.

HIV-1 immunogens typically fail to elicit neutralizing antibody responses to panels of primary isolates [3], [90]–[92]. However, such panels often give little information on the fine specificities of the elicited antibodies that can neutralize a particular isolate. In this study, we used pseudoviruses carrying a number of variant Env proteins to investigate the specificity of rabbit sera that showed strong autologous activity against JR-CSF. The results showed that the C3 and V5, C3 and V4, or V4 regions of Env and specific glycans are largely responsible for this neutralization activity. Antibodies targeting regions in the outer domain of Env have been documented in other studies. The C3, V4, and V5 regions have recently been shown to participate in autologous neutralizing antibody responses that appear during early clade C HIV-1 infections [93], [94]. Binding antibodies to V4 or V5 have been experimentally elicited as evidenced by serum reactivity using immobilized peptides [63], [88], [89], [95], [96], but it is unknown if these are neutralizing antibodies. To our knowledge, this is the first demonstration that neutralizing antibodies can be elicited to these epitopes on a primary isolate by experimental immunization.