Human CD4 binds immunoglobulins

Area of Science:

• Immunology
• Virology
• Molecular Biology

Background:

• T cell glycoprotein CD4 is known to bind to MHC class II molecules and the HIV envelope protein gp120.
• The interaction of CD4 with other molecules is critical for immune responses and viral entry.
• Understanding CD4's binding partners and mechanisms is essential for developing therapeutic strategies against HIV.

Purpose of the Study:

• To investigate the binding of recombinant CD4 (rCD4) to human immunoglobulin (Ig).
• To identify the specific regions and domains of CD4 responsible for Ig binding.
• To explore the functional implications of CD4-Ig interaction in antibody-antigen complex formation and HIV interaction.

Main Methods:

• Binding assays using recombinant CD4 (rCD4) and polyclonal Ig, including human myeloma proteins.
• Characterization of binding dependence on Ig fragments (Fab vs. Fc) and light chain.
• Inhibition studies using soluble rCD4, HIV gp120, and sulfated dextrans.
• Peptide mapping using synthetic peptides to localize the Ig-binding region on CD4.

Main Results:

• Recombinant CD4 (rCD4) demonstrated binding to 78% of tested human myeloma proteins and polyclonal Ig.
• CD4-Ig binding was dependent on the Fab portion of Ig and independent of the light chain.
• The critical region for CD4 binding to Ig was mapped to amino acids 21–38 in the first extracellular domain of CD4.
• CD4-bound antibodies showed a 100-fold enhancement in complexing with antigen compared to antibodies alone.

Conclusions:

• CD4 directly binds to the Fab fragment of immunoglobulin, independent of the light chain.
• The N-terminal region (amino acids 21–38) of CD4 is critical for this interaction.
• CD4-Ig binding enhances antibody-antigen complex formation, potentially contributing to antibody-mediated enhancement of cellular HIV interaction.