HIV-2 neutralization by intact V3-specific Fab fragments

Area of Science:

• Immunology
• Virology
• Molecular Biology

Background:

• The V3 region of human immunodeficiency virus (HIV) surface glycoproteins (gp120 for HIV-1, gp125 for HIV-2) is a key target for neutralizing antibodies.
• Understanding antibody structure-function relationships is critical for developing effective HIV therapies.

Purpose of the Study:

• To characterize conformation-sensitive (3C4) and linear site-specific (7C8) anti-HIV-2 V3 monoclonal antibodies (mAbs) and their Fab fragments.
• To evaluate the neutralization capacity of these antibodies and fragments against HIV-2 isolates.

Main Methods:

• Purification and characterization of anti-HIV-2 V3 monoclonal antibodies (mAbs) and their papain-generated Fab fragments.
• In vitro neutralization assays using HIV-2 isolates, SIVsm, and subtype A and B HIV-2 isolates.
• Sequence analysis of the hypervariable regions, including the third complementarity-determining region of the heavy chain (CDRH3), of the Fab fragments.

Main Results:

• Neither purified 7C8 nor 3C4 mAbs neutralized HIV-2 in vitro.
• Intact Fab fragments from both 7C8 and 3C4 mAbs effectively blocked in vitro infection of HIV-2 isolates.
• The conformation-sensitive 3C4 Fab fragment demonstrated neutralization against both subtype A and B HIV-2 isolates and SIVsm.
• Sequence analysis revealed that the CDRH3 regions of 7C8 and 3C4 Fabs were shorter than those of previously characterized neutralizing antibodies.

Conclusions:

• The larger size of whole monoclonal antibodies (mAbs) likely causes steric hindrance, preventing effective neutralization of HIV-2 by preventing access to the V3 region.
• The smaller size of Fab fragments allows for better access to the V3 epitope on the HIV-2 virion surface, enabling neutralization.
• These findings highlight the importance of antibody fragment size in achieving neutralization of certain viral targets like HIV-2.