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In vitro analysis of complement-dependent HIV-1 cell infection using a model system
P Tacnet-Delorme1, V Boyer, N M Thielens
1Laboratoire d'Enzymologie Moléculaire, Institut de Biologie Structurale, Grenoble, France.
Insights
Human complement (C) enhances HIV-1 infection by activating the classical C pathway. This process involves C3b opsonization and interaction with CD21 and CD4 receptors on cells.
Area of Science:
- Immunology
- Virology
- Biochemistry
Background:
- Previous research indicated human serum (C) enhances HIV-1 cell infection.
- The specific role of complement proteins and mechanisms remained unclear.
Purpose of the Study:
- To differentiate complement from other serum factors in HIV-1 infection enhancement.
- To identify proteins and mechanisms involved in complement-dependent HIV-1 cell infection.
Main Methods:
- Reconstituted the classical complement activation pathway using purified proteins (C1q, C1r, C1s, C4, C2, C3, factor H, factor I).
- Tested the effect of reconstituted complement on HIV-1 infection of MT2 cells.
- Utilized heat-inactivated and depleted sera (C2, C3) to confirm complement's role.
- Investigated the inhibitory effects of CD21 peptides and soluble CD4.
Main Results:
- A mixture of purified complement proteins replicated serum's enhancement of HIV-1 infection at low viral doses.
- The enhancing effect was dependent on functional C2 and C3, and abolished by heat inactivation.
- Both CD21-mimicking peptides and soluble CD4 inhibited the complement-dependent infection.
Conclusions:
- HIV-1 directly activates the classical complement pathway in vitro.
- This activation facilitates HIV-1 infection of MT2 cells, particularly at low virus concentrations.
- The mechanism involves C3b-mediated opsonization, interaction with CD21 receptors, and subsequent entry via CD4 receptors.
Abstract:
Previous studies based on the use of human serum as a source of C have provided evidence for the C-dependent enhancement of cell infection by HIV-1. The present study was undertaken to distinguish C from other serum factors and to identify the proteins and the mechanisms involved in C-dependent cell infection by HIV-1. The classical C activation pathway was reconstituted from the proteins C1q, C1r, C1s, C4, C2, C3, factor H, and factor I; each were purified to homogeneity. A mixture of these proteins at physiological concentrations was shown to reproduce the ability of normal human serum to enhance the infection of MT2 cells by HIV-1 at low doses of virus. This enhancing effect was abolished when heat-inactivated serum and C2- or C3-depleted serum were used, and was restored upon addition of the corresponding purified proteins. A mixture of two synthetic peptides corresponding to positions 10-15 and 90-97 of human C receptor type 2 (CD21) as well as soluble CD4 both inhibited the C-dependent infection process. These data provide unambiguous evidence that HIV-1 triggers a direct activation of the classical C pathway in vitro and thereby facilitates the infection of MT2 cells at low doses of virus. These findings are consistent with a mechanism involving increased interaction between the virus opsonized by C3b-derived fragment(s) and the CD21 cell receptors and subsequent virus entry through CD4 receptors.