Complement enhancement of HIV infection is mediated by complement receptors

Immunopharmacology, 25 (1993) 87-93 Elsevier Science Publishers B.V.

87

I M P H A R 00635

Invited Review

Complement enhancement of HIV infection is mediated by complement receptors Nathalie T h i e b l e m o n t , C a t h e r i n e Delibrias, Elisabeth Fischer, L a u r e n c e Weiss, M i c h e l D. K a z a t c h k i n e a n d Nicole Haeffner-Cavaillon I N S E R M U28, Hdpital Broussais, Paris, France (Accepted 3 November 1992)

Introduction Human immunodeficiency virus (HIV) is tropic and cytopathic for human T lymphocytes bearing the CD4 molecule. In vivo and in vitro findings suggest that HIV tropism is not limited to CD4 positive cells. The identification of the infected target cells and of the mechanisms underlying the viral entry are critical to the understanding of the pathogenesis of this disease, and may facilitate therapeutical approaches. The present review focuses on the enhancing role of complement on infection of CD4 + and CD4 target cells.

Complement activation by HIV Most viruses elicit in man an antiviral humoral immune response. Virus particles coated with immunoglobulin G (IgG) or IgM activate the classical pathway of the complement system through the formation of closely packed IgG clusters and activation of complement component C1. Some viruses also activate complement in the absence of antibodies, the p15E envelope constituant of Moloney leukemia virus has been Correspondence to: M.D. Kazatchkine, I N S E R M U28, Hrpital Broussais, 96 rue Didot, 75014 Paris, France.

identified as a Cl-activating retroviral protein (Bartholomew et al., 1978) whereas Epstein-Barr viruses directly activate the alternative pathway through its envelope glycoprotein gp350 (Mold et al., 1988). The HIV1 efficiently activates complement (Perricone et al., 1987; Senaldi et al., 1990; Tausk et al., 1986), although activation is poorly effective in neutralizing and lysing the virus (Banapour etal., 1986). The molecular mechanisms that are responsible for the resistance of HIV to lysis or inactivation by human complement have not yet been identified. HIV infection is also associated with acquired changes in the membrane expression of complement regulatory molecules, including a decreased expression of CR1 on erythrocytes (Jouvin et al., 1987), a defective expression of DAF(CD55) on leukocytes (Lederman et al., 1989) and of the CD59 membrane inhibitor of complementmediated cytolysis on T lymphocytes (Weiss et al., 1992). The latter alterations could explain the increased susceptibility of lymphocytes of AIDS patients to homologous complementmediated lysis (Lederman et al., 1989). Intact HIV, recombinant gp160, synthetic peptide of gp41 and HIV-infected cells have been shown to activate complement in human serum resulting in the cleavage of C3 (SOlder et al., 1989; Ebenbichler et al., 1991). Complement ac-

88 tivation by HIV occurs through the classical pathway following the binding of C l q to conserved extracellular peptidic sequences of the gp41 envelope protein of HIV-1 and direct activation of the C1 complex. An epitope which contains the major C1 binding and activating sequence is exposed after sCD4 binding to gpl20. This change of the steric configuration of the CD4-gp 120-gp41 complex could facilitate the interaction between gp41 and the C1 complex and could mediate antibody-independent activation of C1 (Ebenbichler et al., 1991). Recent evidence indicates that, upon classical pathway activation in whole serum, mammalian-derived recombinant gpl60 and gpl20 bind C3b/iC3b with a molecular stoichiometry of one to one in the absence of antibodies. Although classical pathway activation of C1 by gp41 and by gpl60 may occur in the absence of antibodies, it was significantly enhanced in serum containing antiHIV antibodies (Thieblemont et al., 1992). As a result of complement activation by viral particles, the infectious agents become opsonized with complement proteins including Clq, C4b and C3b. These proteins promote viral clearance but may also, in some cases prevent or enhance viral attachment to complement receptors on cell surfaces.

Complement-mediated penetration of HIV Complement alone or in association with antiHIV antibodies was shown to enhance infection of various cell targets with opsonized HIV in vitro (Robinson et al., 1988; Tremblay et al., 1990; Boyer et al., 1991; Thieblemont et al., 1992). The ability of HIV to cleave C3 in whole human serum results in the effective binding of C3b/iC3b onto the envelope glycoprotein complex and provides a basis for C3-mediated facilitation of infection with HIV of both CD4-positive and CD4-negative target cells, including T lymphocytes and T cell lines (Robinson et al., 1988; June etal., 1991; Boyer etal., 1991), B cell lines (Tremblay et al., 1990; Gras et al., 1991), mono-

cytes (Thieblemont et al., 1992) monocytic cell lines (Reisinger et al., 1990), and glial cells (Thieblemont et al., 1992).

Complement-mediated enhancement of infection of monocyte/macrophages Instead of destroying HIV in infected individuals, mononuclear phagocytes may serve as a reservoir and a means of transportation of the virus through the human body. Depending on their state of maturation, cells of the monocytic lineage express various amounts of immunoglobulin Fc receptors gamma: RI(CD64), Fc gamma RII(CD32), Fc gamma RIII(CD16), and complement receptors: CR1(CD35) and CR3(CD1 lb/CD18). CR1 is a polymorphic glycoprotein of 160-250 kDa which serves as the receptor for C3b and C4b. CR3 is a heterodimer belonging to the Leu CAM integrin family including the L F A - I ( C D l l a / C D 1 8 ) and p1509 5 ( C D l l c / C D 1 8 ) molecules. The monocytes also express the CD4 molecule at low density on the membrane. In vitro infection of monocytes/macrophages with HIV is enhanced by complement in the presence of antibodies and in the absence of antibodies (Reisinger etal., 1990; Thieblemont et al., 1992). Binding and uptake of purified fluorescein-labeled HTLV-IIIB by the monocytes has been shown to be increased following opsonization of the virus with complement or with antibodies and complement (Bakker et al., 1992). Infection with low amounts of HIV-1 of the promonocytic U937 cells is facilitated by complement without anti-HIV IgG. Infection of U937 cells was reduced in the presence of antibodies directed against the CD4 molecule, or the CR3 receptor, and was abrogated with a combination of the two antibodies (Reisinger, 1990). We have recently demonstrated that opsonization of HIV with complement results in earlier and enhanced infection of cells of the monocytic lineage (THP1, Mono Mac6) with HIV-1 and HIV-2. The enhancing effect was lost if opsonization was performed in the presence of EDTA or with C2 deficient serum. The enhancing effect of complement was

89 totally suppressed by blocking CRl(CD35) or CR3(CDllb/CD18) receptors with anti-CR1 and anti-CR3 F(ab')2 fragments, while blocking the LFA-1 molecule with anti-LFA F(ab')2 fragments had no effect. It may be anticipated that deposition of higher amounts of C3 fragments on the viral envelope in the presence of anti-HIV antibodies and coopsonization of the virus would further enhance the infection of cells of the monocytic lineage. Stimulation of the C3b/C4b receptor (CR1) in serum-free cultured normal human monocytes results in the induction of production of cellassociated I L - l e and fl and in the extracellular release of biologically active IL-lfl (Bacle et al., 1990) and stimulation of CR3 induces the intracellular accumulation of both IL-1 (Couturier et al., 1990). Transcription of the IL-lfl genes as well as HIV replication are dependent on translocation of nuclear factors such as NF-kappaB or AP-1. We have recently demonstrated that stimulation of the CD35 receptor induces a higher viral replication in different monocytic cell lines and a higher translocation of the nuclear transcriptional factor NF-kappaB (Thieblemont et al., manuscript in preparation). Therefore, it can be suggested that during opportunist infections, the benefit of complement activation for efficient killing of bacteria may be counterbalanced by the triggering of complement receptors CR1 and CR3 on monocytes, since the stimulation of CR1 induces translocation of nuclear proteins and contributes to induction of IL-1 gene concomitently to the reactivation of the latent proviruses.

Complement-mediated enhancement of infection of T lymphocytes Approximately 15 ~o of peripheral blood CD4 + and CD8 + T cells express CR1 (Wilson et al., 1983; Rodgaard et al., 1991) and 50~o express CR2 at an approximately 10-fold lower density than B cells (Fischer et al., 1991). CR1 + T lymphocytes also express Fc gamma receptors. CR2 is a 145 kDa glycoprotein that functions as a receptor for C3dg/C3d and iC3b (Cooper

et al., 1988) and as the receptor for EBV on B lymphocytes (Fingeroth et al., 1984; Nemerow et al., 1985). On the human T cell line HPB-ALL, CR2 and CR1 are co-internalized when crosslinked with anti-receptor antibodies; CR2 is capable of signal transduction (Delibrias et al., 1992). The first evidence for the role of complement in enhancing HIV infection o f T cells came from the observation by Robinson that sera from more than 80~o of HIV antibody-positive individuals increased infectious virus release from cells of the MT2 T cell line upon infection with HIV-1 in vitro. Enhancement of CD4 + CR2 + infection by seropositive serum required complement, antiHIV antibodies and the CD4 molecule (Robinson et al., 1988, 1989). We have subsequently found that complement alone was capable of enhancing infection of MT2 cells in the presence of suboptimal amounts of HIV-1 (Boyer et al., 1991). Productive viral infection of MT2 cells with low inputs of HIV occurred through the interaction of opsonized virus with CR2, since infection was blocked by cross-linked mAb against CR2 (Boyer et al., 1991). Complement alone also enhances infection of PHA-stimulated seronegative peripheral blood leukocytes cocultured with cells from HIV-infected individuals (Boyer et al., 1991). Little is known at present on the role of CR1 in facilitating the infection of T cells with complement-opsonized HIV. Preliminary experiments indicate that blocking of CR1 partially decreased CD4-dependent infection of the CD4 + CD8 + CR1 + CR2 + HPB-ALL T cell line with complement-opsonized HIV. CR1 on T cells could either function as a receptor alone mediating viral entry into the cells or as a cofactor for the cleavage of C3b into iC3b and C3dg to allow the interaction of opsonized virus with CR2.

Complement-mediated enhancement of infection of B cells Lymphoblastoid B cell lines express CR2 and, some of the cell lines also express CR1. The

90 expression of CR2 is upregulated by EBV in EBV-transformed B cell lines (Cohen etal., 1987). All normal mature B lymphocytes express CR1 and CR2 (Tedder et al., 1983, 1984). Triggering of CR1 on pre-activated B cells enhances their differentiation into antibody-secreting cells (Daha et al., 1984; Weiss et al., 1987), whereas triggering of CR2 induces B cell proliferation (Bohnsack et al., 1988; Hatzfeld et al., 1988). Early studies have shown that EBV-transformed human B cells lines are susceptible to HIV infection in vitro (Montagnier et al., 1984; Salahuddin et al., 1987). Depending on the cell line and on the strain of HIV, infection of EBV + B cells has resulted in cell lysis or in persistent non-productive or productive infection (Dahl etal., 1987, 1990; Tremblay et al., 1988; De Rossi et al., 1990). HIV has also been shown to infect EBV-Burkitt lymphoma B cell lines, indicating that susceptibility to HIV infection is not strictly dependent on the presence of the EBV genome (Monroe et al., 1988). An enhancing role of complement on infection ofEBV + B cells was first shown in an experimental system wherein CD4 + B lymphoblastoid cell line was used as the target for infection in the presence of complement and anti-HIV antibodies (Tremblay et al., 1990). A recent report indicated that antibody and complement-dependent enhancement of infection of the EBV + IC-1 B cell line was mediated by CR2 and blocked by anti-CD4 antibodies (Gras et al., 1991). Opsonization with complement would enhance infection by increasing the number of viral particles attaching to CR2bearing cells and by subsequently facilitating the entry of the virus through CD4 on cells expressing low amounts of the molecule. CR2 may mediate on its own the productive infection of the Raji B lymphoblastoid cell line with complement-opsonized H1V (Boyer et al., 1992). Infection of the cells occurred independently of antibodies and of CD4 molecule since HIV-seronegative serum was used for infection and since the cells lack the expression of CD4 surface antigen and of CD4 transcript.

CD4-independent productive HIV infection of human cells The CD4 molecule mediates the high affinity binding of the gpl20 envelope glycoprotein of HIV to target cells expressing CD4 (Mizukami et al., 1988; Sattentau et al., 1986). Recent studies have shown that a number of C D 4 - cells are susceptible to HIV infection in vivo and in vitro, indicating that alternative pathways to CD4 exist for viral penetration into target cells. HIV infection of human neural cells and primary fetal dorsal root ganglia glial cells involved an entry pathway distinct from that of lymphoid cells. The medulloblastoma and glioblastoma cells had no detectable CD4 on their surfaces and nor detectable transcription of the CD4 gene using either RNA blot hybridization or nuclease protection assays but were infectable by various isolates of HIV (Harouse etal., 1989). The infection of human fibroblastoid cells (Tateno et al., 1989), clonal and purified polyclonal populations of natural killer cells (Chehimi etal., 1991), hepatoma cells (Cao et al., 1990), transformed trophoblast-derived cells (Zachar et al., 1991) and Raji B lymphoblastoid cell line (Boyer et al., 1992) which expressed neither surface CD4 nor CD4 m R N A is not blocked by anti-CD4 monoclonal antibodies. Furthermore, HIV-1 infection in hepatoma and neuronal cell lines could not be blocked soluble CD4 (Li et al., 1990). Similar results were obtained using human follicular dendritic cells (Stahmer etal., 1991), brain and muscle cells (Clapham et al., 1990) that could be infected with HIV-1. All these cells were infected in the presence of soluble CD4 or anti-CD4 antibody Leu-3a. Monocytes/macrophages, whether freshly isolated or two months old, are susceptible to HIV infection in spite of the fact that CD4 + molecule are not detectable on macrophages after 15 days of culture (Valentin et al., 1990). We have observed that CD4 + THP1 promonocytic cells and the monocytic CD4 Mono Mac6 and glial U251-MG cell lines which lack expression of CD4 m R N A and of the CD4 antigen are similarly susceptible to HIV-1 and HIV-2 infection in the presence of serum (Thie-

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blemont et al., 1992). Complement-mediated infection of mononuclear phagocytic cells by HIV may occur independently of CD4, indicating that the interaction of opsonized virus with complement receptors may be sufficient to mediate penetration of HIV into monocytes. As in the case of other infectious microorganisms, complement activation may contribute to the pathogenesis of HIV infection by opsonizing the virus and facilitating entry of viral particles. Therefore in addition to attachment of HIV-1 to Fc-receptor via antibodies, gangliosides and LFA-1 molecule, C3 complement receptors may contribute, alone or in association with CD4, to infection of various cells with opsonized HIV.

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