Complement-mediated enhancement of HIV-1 infection in peripheral blood mononuclear cells

HIV-1 Trans Infection via TNTs Is Impeded by Targeting C5aR

Nonadjacent immune cells communicate through a complex network of tunneling nanotubes (TNTs). TNTs can be hijacked by HIV-1, allowing it to spread between connected cells. Dendritic cells (DCs) are among the first cells to encounter HIV-1 at mucosal sites, but they are usually efficiently infected only at low levels. However, HIV-1 was demonstrated to productively infect DCs when the virus was complement-opsonized (HIV-C). Such HIV-C-exposed DCs mediated an improved antiviral and T-cell stimulatory capacity. The role of TNTs in combination with complement in enhancing DC infection with HIV-C remains to be addressed. To this aim, we evaluated TNT formation on the surface of DCs or DC/CD4+ T-cell co-cultures incubated with non- or complement-opsonized HIV-1 (HIV, HIV-C) and the role of TNTs or locally produced complement in the infection process using either two different TNT or anaphylatoxin receptor antagonists. We found that HIV-C significantly increased the formation of TNTs between DCs or DC/CD4+ T-cell co-cultures compared to HIV-exposed DCs or co-cultures. While augmented TNT formation in DCs promoted productive infection, as was previously observed, a significant reduction in productive infection was observed in DC/CD4+ T-cell co-cultures, indicating antiviral activity in this setting. As expected, TNT inhibitors significantly decreased infection of HIV-C-loaded-DCs as well as HIV- and HIV-C-infected-DC/CD4+ T-cell co-cultures. Moreover, antagonizing C5aR significantly inhibited TNT formation in DCs as well as DC/CD4+ T-cell co-cultures and lowered the already decreased productive infection in co-cultures. Thus, local complement mobilization via DC stimulation of complement receptors plays a pivotal role in TNT formation, and our findings herein might offer an exciting opportunity for novel therapeutic approaches to inhibit trans infection via C5aR targeting.

Viral Evasion of the Complement System and Its Importance for Vaccines and Therapeutics

The complement system is a key component of innate immunity which readily responds to invading microorganisms. Activation of the complement system typically occurs via three main pathways and can induce various antimicrobial effects, including: neutralization of pathogens, regulation of inflammatory responses, promotion of chemotaxis, and enhancement of the adaptive immune response. These can be vital host responses to protect against acute, chronic, and recurrent viral infections. Consequently, many viruses (including dengue virus, West Nile virus and Nipah virus) have evolved mechanisms for evasion or dysregulation of the complement system to enhance viral infectivity and even exacerbate disease symptoms. The complement system has multifaceted roles in both innate and adaptive immunity, with both intracellular and extracellular functions, that can be relevant to all stages of viral infection. A better understanding of this virus-host interplay and its contribution to pathogenesis has previously led to: the identification of genetic factors which influence viral infection and disease outcome, the development of novel antivirals, and the production of safer, more effective vaccines. This review will discuss the antiviral effects of the complement system against numerous viruses, the mechanisms employed by these viruses to then evade or manipulate this system, and how these interactions have informed vaccine/therapeutic development. Where relevant, conflicting findings and current research gaps are highlighted to aid future developments in virology and immunology, with potential applications to the current COVID-19 pandemic.

A Novel Role for the Receptor of the Complement Cleavage Fragment C5a, C5aR1, in CCR5-Mediated Entry of HIV into Macrophages

The complement system is an ancient pattern recognition system that becomes activated during all stages of HIV infection. Previous studies have shown that C5a can enhance the infection of monocyte-derived macrophages and T cells indirectly through the production of interleukin (IL)-6 and tumor necrosis factor (TNF)-α and the attraction of dendritic cells. C5a exerts its multiple biologic functions mainly through activation of C5a receptor 1 (C5aR1). Here, we assessed the role of C5aR1 as an enhancer of CCR5-mediated HIV infection. We determined CCR5 and C5aR1 heterodimer formation in myeloid cells and the impact of C5aR1 blockade on HIV entry and genomic integration. C5aR1/CCR5 heterodimer formation was identified by immunoprecipitation and western blotting. THP-1 cells and monocyte-derived macrophages (MDM) were infected by R5 laboratory strains or HIV pseudotyped for the vesicular stomatitis virus (VSV) envelope. Levels of integrated HIV were measured by quantitative PCR after targeting of C5aR1 by a C5aR antagonist, neutralizing C5aR1 monoclonal antibody (mAb) or hC5a. C5aR1 was also silenced by specific siRNA prior to viral entry. We found that C5aR1 forms heterodimers with the HIV coreceptor CCR5 in myeloid cells. Targeting C5aR1 significantly decreased integration by R5 viruses but not by VSV-pseudotyped viruses, suggesting that C5aR1 is critical for viral entry. The level of inhibition achieved with C5aR1-blocking reagents was comparable to that of CCR5 antagonists. Mechanistically, C5aR1 targeting decreased CCR5 expression. MDM from CCR5Δ32 homozygous subjects expressed levels of C5aR1 similar to CCR5 WT individuals, suggesting that mere C5aR1 expression is not sufficient for HIV infection. HIV appeared to preferentially enter THP-1 cells expressing high levels of both C5aR1 and CCR5. Targeted reduction of C5aR1 expression in such cells reduced HIV infection by ~50%. Our data thus suggest that C5aR1 acts as an enhancer of CCR5-mediated HIV entry into macrophages, the targeting of which may prove useful to reduce HIV infection by R5 strains.

Follicular Dendritic Cells Retain Infectious HIV in Cycling Endosomes

Despite the success of antiretroviral therapy (ART), it does not cure Human Immunodeficiency Virus (HIV) and discontinuation results in viral rebound. Follicular dendritic cells (FDC) are in direct contact with CD4+ T cells and they retain intact antigen for prolonged periods. We found that human FDC isolated from patients on ART retain infectious HIV within a non-degradative cycling compartment and transmit infectious virus to uninfected CD4 T cells in vitro. Importantly, treatment of the HIV+ FDC with a soluble complement receptor 2 purges the FDC of HIV virions and prevents viral transmission in vitro. Our results provide an explanation for how FDC can retain infectious HIV for extended periods and suggest a therapeutic strategy to achieve cure in HIV-infected humans.

Human immunodeficiency virus (HIV) can lead to acquired immunodeficiency syndrome, or AIDS. Before the introduction of anti retroviral therapy (ART) in the mid-1990s, people with HIV could progress to AIDS in just a few years. Today patients with HIV have a close to normal life expectancy. Worldwide, there are about 2 million new cases of HIV per year. Currently about 35 million people are living with HIV of which around 13 million receive ART. Still an estimated 1.5 million people die from the consequences of HIV each year. Despite the success of ART, it does not cure HIV and discontinuation results in viral rebound. Follicular dendritic cells (FDC), located central to the B cell follicle, are also in direct contact with T cells. FDCs retain intact antigen for prolonged periods. We found that human FDCs isolated from patients on ART retain infectious HIV and can transmit virus to uninfected T cells in vitro. Treatment of the HIV+ FDC with a soluble complement receptor 2 purges the FDC of HIV virions and prevents viral transmission to T cells in vitro. Our results can explain how FDCs retain infectious HIV and suggest a therapeutic strategy to come closer to a cure.

Follicular dendritic cells: dynamic antigen libraries

Follicular dendritic cells (FDCs) are essential for high-affinity antibody production and for the development of B cell memory. Historically, FDCs have been characterized as 'accessory' cells that passively support germinal centre (GC) responses. However, recent observations suggest that FDCs actively shape humoral immunity. In this Review, we discuss recent findings concerning the antigen acquisition and retention functions of FDCs, and relevant implications for protective immunity. Furthermore, we describe the roles of FDCs within GCs in secondary lymphoid organs and discuss FDC development within this dynamic environment. Finally, we discuss how a better understanding of FDCs could facilitate the design of next-generation vaccines.

The lectin pathway of complement: advantage or disadvantage in HIV pathogenesis?

The pattern recognition molecules of the lectin complement pathway are important components of the innate immune system with known functions in host-virus interactions. This paper summarizes current knowledge of how these intriguing molecules, including mannose-binding lectin (MBL), Ficolin-1, -2 and -3, and collectin-11 (CL-11) may influence HIV-pathogenesis. It has been demonstrated that MBL is capable of binding and neutralizing HIV and may affect host susceptibility to HIV infection and disease progression. In addition, MBL may cause variations in the host immune response against HIV. Ficolin-1, -2 and -3 and CL-11 could have similar functions in HIV infection as the ficolins have been shown to play a role in other viral infections, and CL-11 resembles MBL and the ficolins in structure and binding capacity.

The good and evil of complement activation in HIV-1 infection

The complement system, a key component of innate immunity, is a first-line defender against foreign pathogens such as HIV-1. The role of the complement system in HIV-1 pathogenesis appears to be multifaceted. Although the complement system plays critical roles in clearing and neutralizing HIV-1 virions, it also represents a critical factor for the spread and maintenance of the virus in the infected host. In addition, complement regulators such as human CD59 present in the envelope of HIV-1 prevent complement-mediated lysis of HIV-1. Some novel approaches are proposed to combat HIV-1 infection through the enhancement of antibody-dependent complement activity against HIV-1. In this paper, we will review these diverse roles of complement in HIV-1 infection.

Complement and its role in protection and pathogenesis of flavivirus infections

The complement system is a family of serum and cell surface proteins that recognize pathogen-associated molecular patterns, altered-self ligands, and immune complexes. Activation of the complement cascade triggers several antiviral functions including pathogen opsonization and/or lysis, and priming of adaptive immune responses. In this review, we will examine the role of complement activation in protection and/or pathogenesis against infection by Flaviviruses, with an emphasis on experiments with West Nile and Dengue viruses.

HIV and complement: hijacking an immune defense

The complement system is a central player of the innate immune system. Activation of the complement system protects the host against pathogens. However, uncontrolled synthesis can be detrimental to host. This concise review summarizes the current understanding of the mechanism(s) of complement activation, the mechanism of C3 regulation, and the role of complement in human immunodeficiency virus (HIV) pathogenesis with emphasis on the cross-talk between HIV and complement system in NeuroAIDS and HIV-associated nephropathy (HIVAN).

Comparison of human immunodeficiency virus (HIV)-specific infection-enhancing and -inhibiting antibodies in AIDS patients

The humoral immune response of the human host against the human immunodeficiency virus (HIV) type 1 (HIV-1) envelope glycoproteins comprises virus-neutralizing antibodies (NAs), antibody-dependent cellular cytotoxicity-mediating (ADCC) antibodies, and infection-enhancing antibodies (IEAs). Because of their potential significance for the outcome of infection with this virus, we have studied the relative prevalence of NAs, ADCC antibodies, and IEAs in the sera of patients infected with HIV. Our results demonstrate that while >or=60% of serum samples are positive for NAs or ADCC antibodies, 72% of these serum samples mediate the enhancement of infection in the presence of complement. In patients with low CD4 counts, NA and ADCC antibody levels tend to decrease, while IEA levels increase. A significant positive correlation was found only between the presence of ADCC antibodies and the presence of antibodies that neutralized HIV-1 in the presence of complement. These results show that the anti-HIV-1 humoral immune response consists of a mixture of antibodies that may inhibit or enhance HIV infection and whose ratios may vary in different stages of the infection.

Strong correlation between the complement-mediated antibody-dependent enhancement of HIV-1 infection and plasma viral load

OBJECTIVE

We have previously demonstrated that complement-mediated antibody-dependent enhancement (C-ADE) of HIV-1 infection correlates with accelerated immunosuppression and disease progression in HIV-1-infected individuals. In the present work the relationship between C-ADE and plasma HIV-1 RNA concentrations was studied to determine the effect of C-ADE on viral replication.

METHODS

Three studies were performed: (a) C-ADE and HIV-1 RNA concentrations were determined in the serum and plasma aliquots taken at the same time from 98 HIV patients, mostly in the advanced stage of the disease; (b) the above two parameters as well as HIV enzyme-linked immunosorbent assay (ELISA)-reactive antibodies (Abbott HIV 1/2 test), and p24 antigen levels (Abbott antigen test; Abbott, Delkenheim, Germany) were determined in four seroconversion panels purchased from the Boston Biomedica firm; (c) changes of HIV-1 RNA concentration and C-ADE during a 17 month follow-up period were determined in 18 HIV-infected patients. C-ADE was measured by the method previously established in our laboratories. The results were expressed by an enhancement/neutralization index (E/NI). HIV-1 RNA levels were determined with the Amplicor monitor kit (Roche, Basel, Switzerland), and in some experiments with the nucleic acid sequence based amplification (Organon Teknika, Turnhout, Belgium) kits.

RESULTS

(a) We found a highly significant (P<0.0001) positive correlation between E/NI values reflecting the extent of HIV-1 infection enhancement and plasma HIV-1 RNA levels. Both E/NI and HIV-1 RNA levels negatively correlated to the CD4 cell counts. (b) C-ADE was first detected just before, or concomitantly with, seroconversion in 4/4 seroconversion panels. (c) Both E/NI values and HIV-1 RNA levels significantly (P<0.001) increased during a 17 month observation period in 18 HIV-infected patients.

CONCLUSION

We found strong association between the extent of the complement-mediated antibody-dependent enhancement of HIV-1 infection and the plasma viral load in HIV patients. On the basis of these findings, C-ADE correlates with HIV replication in vivo, and potentially contributes to the progression of HIV disease.