Introduction: 2F5 is a rare, neutralizing human mAb which is specific for the HIV gp41 membrane proximal region (MPER) and exhibits lipid-reactivity. A major goal of HIV vaccine research is to understand why neutralizing antibodies like 2F5 cannot be elicited in response to immunization or through the course of HIV infection.

 

 

 Objectives: To identify MPER antibody and B cell responses in naïve and 2F5 peptide-immunized mice and to determine if 2F5-like antibodies are controlled by B cell immunoregulatory mechanisms.

 

 

 Methods: BALB/c mice were immunized X5 with an HIV-1 2F5 peptide (DP178/T20, spanning the MPER HR2 region). Naïve or immunized mice were screened for serum 2F5 antibodies by ELISA and for 2F5 antigen-specific B cells using 2F5 epitope-specific tetramers.

 

 

 Results: Immunization with DP178/T20 induced non-neutralizing MPER antibodies in mice. In naïve, unimmunized BALB/c mice, the 2F5 epitope (but not a control V3 gp120 epitope) bound to the sIgs of a significant (~5-7%) subset of B220+ B cells, and this binding mapped to the IgHa heavy chain allotype. Furthermore, DP178/T20 peptide immunization selectively expanded this 2F5 tetramer-reactive subset of B cells to ~30-50% in bone marrow, spleen, and blood, suggestive that the 2F5 MPER epitope may be acting as a B cell superantigen (sAg). Consistent with properties of known B cell sAgs, 2F5 peptide immunization also resulted in a decreased peritoneal B1a B cell population, increased re-circulating mature follicular bone marrow B2 B cells, a progressive shift to sIgG, then sIgE-expressing polyclonally-expanded B220+ populations over the course of immunization, and a polyfunctional (IL-6, IL-10, and IL-13) plasma cytokine burst.

 

 

 Conclusions: Characterization of the mechanisms involved in this 2F5 epitope-elicited B cell sAg activity may help in delineating why neutralizing 2F5-like anti-MPER Ab induction is not achieved. Specifically, 2F5 epitope sAg stimulation may divert or delete desired antigen reactive B cells. Furthermore, identification of the gp41 MPER residues and IgHa allotypic determinants required for this activity may have important implications for the rational design of gp41 MPER immunogens.