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Binding of recombinant HIV coat protein gp120 to human monocytes
D S Finbloom1, D L Hoover, M S Meltzer
1Division of Cytokine Biology, Food and Drug Administration, Bethesda, MD 20892.
Insights
This study investigates how the HIV envelope protein gp120 binds to monocytes, finding that while direct CD4 binding is limited on cultured monocytes, a CD4-like molecule facilitates virus entry and replication, impacting HIV infectivity.
Area of Science:
- Immunology
- Virology
- Cell Biology
Background:
- The interaction between HIV and macrophages is crucial for viral pathogenesis.
- The expression and role of CD4 receptors on macrophages remain debated.
- Understanding gp120 binding to monocytes is key to deciphering HIV-macrophage interactions.
Purpose of the Study:
- To analyze the binding characteristics of the HIV envelope protein gp120 to freshly isolated and cultured human monocytes.
- To investigate the role of the CD4 molecule in gp120 binding to monocytes.
- To determine the implications of gp120-monocyte interactions for HIV infectivity.
Main Methods:
- Isolation and purification of human monocytes from healthy volunteers.
- Culture of monocytes with macrophage colony-stimulating factor (MCSF).
- Binding assays using iodinated recombinant gp120 (rgp120) and inhibition studies with anti-CD4 antibody (OKT4a) and soluble recombinant CD4 (rCD4).
Main Results:
- Cultured monocytes exhibited significantly lower specific binding of rgp120 compared to MOLT/4 T cells and U937 myeloid cells.
- Anti-CD4 antibody and soluble rCD4 showed minimal inhibition of rgp120 binding to cultured monocytes, unlike their effect on T cells and U937 cells.
- Despite limited direct CD4 binding, a portion of rgp120 binding to cultured monocytes was inhibited by rCD4, suggesting interaction with a CD4-like molecule.
- HIV infectivity was inhibited by concentrations of OKT4a similar to those affecting T cell binding.
Conclusions:
- Most gp120 binding to cultured monocytes occurs independently of the canonical CD4 determinant.
- A CD4-like molecule on monocytes mediates partial gp120 binding, contributing to virus entry and replication.
- These findings highlight distinct mechanisms of HIV interaction with monocytes compared to T cells, influencing viral spread.
Abstract:
Inasmuch as the exact level of CD4 Ag expression on macrophages is controversial and because HIV may interact with macrophages in a manner different from that on T cells, we analyzed the binding of gp120 to freshly isolated and cultured monocytes. rgp120 was iodinated using the lactoperoxidase method to a sp. act. of 600 Ci/mmol. Highly purified monocytes (greater than 90%) were isolated from the leukapheresed blood of normal volunteers by Ficoll-Hypaque sedimentation followed by countercurrent centrifugal elutriation and cultured 7 days in DMEM supplemented with 1000 U/ml macrophage CSF in 10% human serum. Whereas MOLT/4 cells consistently bound freshly prepared 125I-rgp120 at 80% specificity with 5100 +/- 700 mol/cell, MCSF cultured monocytes bound rgp120 at only 0 to 20% specificity and 420 +/- 200 mol/cell. Most of the radioactivity bound by these cells could not be blocked by the addition of unlabeled rgp120. In contrast, the U937 myeloid cell line bound rgp120 with 50% specificity and about 2500 mol/cell. Whereas the antibody OKT4a (anti-CD4) blocked 80% of the binding on MOLT/4 cells and 50% on U937 cells, binding was only inhibited on the average of 6% on cultured monocytes. When soluble rCD4 was used as an inhibitor, binding to MOLT/4 cells was blocked by 80%. In contrast, binding to cultured monocytes was inhibited by 28%. HIV infectivity was blocked by similar concentrations of OKT4a. These observations suggest that although most binding of gp120 to cultured monocytes is not to the CD4 determinant, several hundred molecules do bind to a CD4-like molecule which promotes virus entry and replication.
