HIV-1 gp41 binding proteins and antibodies to gp41 could inhibit enhancement of human Raji cell MHC class I and II expression by gp41

Inhibitory receptor CD47 binding to plasma TSP1 suppresses NK-cell IFN-γ production via activating the JAK/STAT3 pathway during HIV infection

BACKGROUND

Natural killer (NK) cells play an important first-line role against tumour and viral infections and are regulated by inhibitory receptor expression. Among these inhibitory receptors, the expression, function, and mechanism of cluster of differentiation 47 (CD47) on NK cells during human immunodeficiency virus (HIV) infection remain unclear.

METHODS

Fresh peripheral blood mononuclear cells (PBMCs) were collected from people living with HIV (PLWH) and HIV negative controls (NC) subjects. Soluble ligand expression levels of CD47 were measured using ELISA. HIV viral proteins or Toll-like receptor 7/8 (TLR7/8) agonist was used to investigate the mechanisms underlying the upregulation of CD47 expression. The effect of CD47 on NK cell activation, proliferation, and function were evaluated by flow cytometry. RNA-seq was used to identify downstream pathways for CD47 and its ligand interactions. A small molecule inhibitor was used to restore the inhibition of NK cell function by CD47 signalling.

RESULTS

CD47 expression was highly upregulated on the NK cells from PLWH, which could be due to activation of the Toll-like receptor 7/8 (TLR7/8) pathway. Compared with NC subjects, PLWH subjects exhibited elevated levels of CD47 ligands, thrombospondin-1 (TSP1), and counter ligand signal regulatory protein-α (SIRPα). The TSP1-CD47 axis drives the suppression of interferon gamma (IFN-γ) production and the activation of the Janus kinase signal transducer and activator of transcription (JAK-STAT) pathway in NK cells. After treatment with a STAT3 inhibitor, the NK cells from PLWH showed significantly improved IFN-γ production.

CONCLUSIONS

The current data indicate that the binding of the inhibitory receptor CD47 to plasma TSP1 suppresses NK cell IFN-γ production by activating the JAK/STAT3 pathway during HIV infection. Our results suggest that CD47 and its related signalling pathways could be targets for improving NK cell function in people living with HIV.

Generation of HIV-1 potent and broad neutralizing antibodies by immunization with postfusion HR1/HR2 complex

BACKGROUND

The envelope glycoproteins are major targets for HIV vaccines. The N-terminal and the C-terminal regions of the gp41 interact to form six helix bundles that are responsible for the fusion between the viral and the target cell membranes. Monoclonal antibodies (Abs) able to disrupt the formation of this complex or to interfere with it could inhibit HIV fusion. Most of the well described gp41-specific broadly neutralizing Abs target conformational epitopes within the membrane proximal region of gp41 (MPER) and recognize linear peptides.

METHOD AND RESULTS

In this study, a stable human transfected cell line, expressing a well folded heptad repeat regions 1 (HR1)/HR2 postfusion complex was developed. Transfected cells were highly immunogenic in mice and allowed the generation of 40 complex specific B-cell clones. Three of them were able to neutralize efficiently both HIV-1 laboratory adapted virus and primary isolates from different clades. Two neutralizing Abs (Nabs) FC-2 and FC-3 bound to a recombinant folded gp140 and blocked with a high potency HR1/HR2 fusion complex formation in vitro. The conformational epitopes of the three antibodies are different to 2F5, 4E10, D5 or NC-1 and mainly located in the MPER region. Abs were capable of inhibiting syncytium formation by blocking spatial interactions between HR1 and HR2 regions.

CONCLUSION

These findings suggest that immunogenicity of gp41 is achievable and that the use of a fusion complex expressing human cell line is a highly potent immunogen to generate neutralizing antibodies against gp41 envelope glycoprotein.

Cytokine and chemokine responses to selected early secreted antigenic target-6 and culture filtrate protein-10 peptides in tuberculosis

Cytokine [tumor necrosis factor-α, interleukin-2 (IL-2), IL-4] and chemokine [regulated upon activation normal T cell expressed and secreted (RANTES), monocyte chemoattractant protein-1] responses to selected early secreted antigenic target-6 (ESAT-6) and culture filtrate protein-10 (CFP-10) peptides were studied in healthy household contacts and patients with pulmonary tuberculosis (PTB). It was observed that Th1 cytokines and chemokine RANTES positive T cells were elevated in response to the peptides Esp1, Esp6, Cfp6, and Cfp8 in healthy household contacts. IL-4 positive T cells were enhanced by Esp1 and Esp6 in PTB. Monocyte chemoattractant protein-1 positive monocytes increased in response to the peptides Esp1, Esp6, Cfp8, and Cfp9 in PTB. These peptides deserve attention for further immune studies.

Characterization of interaction between C-domain on HIV-1 gp41 and the putative receptor protein p62

Based on the fact that the binding of HIV-1 gp41 to the putative cellular receptor protein p62 could be inhibited by the C-domain peptide of gp41, we wanted to confirm the interaction of the C-domain with p62 and to characterize the receptor-binding site on the C-domain. We attempted to isolate the putative receptor protein p62 from cell lysates of the human B cell line Raji by affinity chromatography using sepharose-columns which were conjugated with three different peptides of the C-domain respectively. A protein of 62 kDa was isolated by peptide (NP2)-sepharose-column, while none of the proteins was identified in eluates of the other two overlapped peptides of C-domain, indicating that HIV-1 gp41 by the region aa635-664 of C-domain binds to the putative receptor protein p62. Besides, CD spectroscopy analysis revealed that only a NP2 peptide could induce significant conformational change of P62. In addition, the interaction between P62 and three peptides of the C-domain was characterized by the surface plasma resonance (SPR) measurement. It was indicated that only the NP2 peptide significantly inhibited the interaction between rsgp41 and the putative receptor P62, confirming that the protein p62 may serve as a potential receptor for gp41 binding, and the peptide NP2 contains an integrate binding site for gp41 binding to p62.

Class I major histocompatibility complex anchor substitutions alter the conformation of T cell receptor contacts

An immunogenic peptide (GP2) derived from HER-2/neu binds to HLA-A2.1 very poorly. Some altered-peptide ligands (APL) of GP2 have increased binding affinity and generate improved cytotoxic T lymphocyte recognition of GP2-presenting tumor cells, but most do not. Increases in binding affinity of single-substitution APL are not additive in double-substitution APL. A common first assumption about peptide binding to class I major histocompatibility complex is that each residue binds independently. In addition, immunologists interested in immunotherapy frequently assume that anchor substitutions do not affect T cell receptor contact residues. However, the crystal structures of two GP2 APL show that the central residues change position depending on the identity of the anchor residue(s). Thus, it is clear that subtle changes in the identity of anchor residues may have significant effects on the positions of the T cell receptor contact residues.

Structural and functional identification of major histocompatibility complex class I-restricted self-peptides as naturally occurring molecular mimics of viral antigens. Possible role in CD8+ T cell-mediated, virus-induced autoimmune disease

Structural similarity (molecular mimicry) between viral epitopes and self-peptides can lead to the induction of autoaggressive CD4(+) as well as CD8(+) T cell responses. Based on the flexibility of T cell receptor/antigen/major histocompatibility complex recognition, it has been proposed that a self-peptide could replace a viral epitope for T cell recognition and therefore participate in pathophysiological processes in which T cells are involved. To address this issue, we used, as a molecular model of viral antigen, the H-2D(b)-restricted immunodominant epitope nucleoprotein (NP)-(396-404) (FQPQNGQFI) of lymphocytic choriomeningitis virus (LCMV). We identified peptide sequences from murine self-proteins that share structural and functional homology with LCMV NP-(396-404) and that bound to H-2D(b) with high affinity. One of these self-peptides, derived from tumor necrosis factor receptor I (FGPSNWHFM, amino acids 302-310), maintained LCMV-specific CD8(+) T cells in an active state as observed both in vitro in cytotoxic assays and in vivo in a model of virus-induced autoimmune diabetes, the rat insulin promoter-LCMV NP transgenic mouse. The natural occurrence and molecular concentration at the surface of H-2(b) spleen cells of tumor necrosis factor receptor I-(302-310) were determined by on-line micro-high pressure liquid chromatography/mass spectrometry and supported its biological relevance.

HIV-1 gp41 and type I interferon: sequence homology and biological as well as clinical implications

HIV-1 gp41-like human type I interferon (IFN) could inhibit lymphocyte proliferation and up-modulate MHC class I and II and ICAM-1 molecule expression. Sequence comparison indicates that a similar epitope RILAV-YLKD exists between N-domain of gp41 and two regions in IFN-alpha(aa29-35 and 113-129), IFN-beta (aa31-37 and 125-138) and IFN-omega (aa29-35 and 123-136), which was shown to form IFN-alpha/beta-receptor binding site. Weak sequence similarity was also found to exist in both regions on gp41 and type I IFN of murine and bovine. Experimental studies indicated that a common immunological epitope exists between gp41 and IFN-alpha and -beta. Antibodies against human IFN-alpha and -beta recognized the common immunological epitope and inhibited gp41-binding to the potential cellular receptor protein p45. Moreover, the polyclonal antibody to IFN-beta completely inhibited gp41-binding to human T, B cells and monocytic cells, while IFN-alpha could only inhibit this binding incompletely. It was interestingly observed that human IFN-beta after preincubating with cells could incompletely inhibit the binding of gp41 to human B cells and monocytic cells, and very weakly inhibit the binding to human T cells, indicating that the receptor for IFN-beta-binding may be involved in gp41 binding. This potential relationship may be based on the amino acid sequence homology in the receptor binding region between gp41 and IFN-beta. It was observed that the increased levels of antibodies against human IFN-alpha and -beta exist in HIV-1-infected individuals and are associated with the common epitope on gp41. Besides, several studies provided experimental evidence that the common immunological epitope could induce protective activity against HIV-1. The IFN-alpha-based vaccine has showed a significant reduction of disease progression in IFN-alpha-vaccine-treated HIV-infected patients. Recent experimental evidence indicates that gp41 and IFN-beta were involved in downregulation of CCR5 expression and induction of cell activation or signal transduction. Whether it may be performed by a similar mechanism is still to be investigated.

HIV-1 gp41 by N-domain binds the potential receptor protein P45

Recent crystal structure analysis of HIV-1 gp41 revealed that two domains (N- and C-domains) on gp41 play an important role in mediating membrane fusion and HIV-1 entry. The experimental evidence that gp41 by N-domain bound the potential receptor protein P45 could help to understand the mechanism of HIV entry. A recombinant soluble gp41 (rsgp41: Env aa539-684) could bind to P45 in the affinity capillary electrophoresis analysis and the surface plasmon resonance assay. In a blockade assay, peptide P1 (Env aa583-599) could inhibit interaction between rsgp41 and P45, while a control peptide could not. Direct binding of rsgp41, rgp41DP (aa567-648), P1 peptide and (P1)(2) peptide [(aa586-596)(2)] to P45 was examined in an ELISA assay. Rsgp41 bound the potential receptor protein P45 strongly, while rgp41DP and P1 as well as (P1)(2) could all weakly bind to P45, indicating that gp41 by N-domain weakly binds P45 and the region RILAVERYLKD located in the N-domain is defined as the binding site for P45 binding.

The C domain of HIV-1 gp41 binds the putative cellular receptor protein P62

OBJECTIVE

To characterize the binding of gp41 with the putative receptor protein P62.

DESIGN

HIV-1 gp41 binds several cellular proteins by two binding sites, one of which has been shown to bind to a putative receptor protein P45 (45 kDa). Based on this, an attempt was made to determine the relationship between the two binding sites and P62 (62 kDa).

METHODS

Using surface plasmon resonance (SPR) measurement, the interaction was measured between recombinant soluble gp41 (rsgp41, Env aa539-684) and protein P62. Inhibition of this interaction was attempted by the use of synthetic peptides (P1, aa583-599; P2, aa646-674) corresponding to the two binding sites in gp41. In addition, the direct binding of P62 to peptide P2 was examined in an enzyme-linked immunosorbent assay.

RESULTS

Using SPR measurement, the interaction between P62 and rsgp41 was confirmed, and the interaction was found to be inhibited by only the synthetic peptide P2 sequence that corresponds to the C domain of gp41; neither P1 nor a control peptide inhibited the interaction. Moreover, like rsgp41, P2 was able to bind P62 whereas P1 and another recombinant gp41 (aa567-648 that does not include the C domain) were not.

CONCLUSIONS

P62 bound rsgp41 and the synthetic peptide P2. This interaction could be inhibited only by P2. These results indicate that the C domain of HIV-1 gp41 binds P62.

A common immunological epitope existing between HIV-1 gp41 and human interferon-alpha and -beta

Based on the similar effects that HIV-1 gp41 like human type I interferons (IFN-alpha and -beta) upregulated MHC expression and inhibited lymphocyte proliferation, we compared amino acid sequences of gp41 and human type I interferons (IFN-alpha and -beta), and found sequence-similarity existing between the immunosuppressive domain (aa583-599, LQARILAVERYLKDQQL) of HIV-1 gp41 and human IFN-alpha (region aa117-129) and IFN-beta (region aa128-134); besides, the immunosuppressive domain of HIV-1 gp41 shows sequence-homology with bovine and murine IFN-alpha and IFN-beta in the same region. We examined polyclonal antibodies to human IFN-alpha and IFN-beta respectively and found that both antibodies could recognize rsgp41 from two different sources. The polyclonal antibody to IFN-beta could bind to the immunosuppressive peptide (ISP, aa583-599, LQARILAVERYLKDQQL) of HIV-1 gp41, only when the ISP was coupled with carrier-protein, and inhibit, if preincubated with rsgp41, binding of rsgp41 to human H9 (T cells), Raji (B cells), and U937 (monocyte cells) completely. The polyclonal antibody to IFN-alpha could partially inhibit the binding of rsgp41 to U937 and Raji. These results indicate that a common immunological epitope exists between HIV-1 gp41 and human type I interferons.

HIV-1 gp41 by a common immunological epitope induces increased levels of antibodies against human interferon-beta in HIV-1 positive individuals

Based on our finding that a similar epitope exists between human IFN-beta (aa128-134) and HIV-1 gp41 (aa586-595), we examined 20 sera from healthy and 20 from HIV-1 infected individuals for IFN-beta antibody levels by ELISA. The levels of anti-IFN-beta antibody in sera from HIV-infected individuals were increased by about 160% in comparison with HIV-negative. We affinity-purified anti-gp41 antibodies from sera of HIV-1-infected individuals using rsgp41-sepharose column. One of three antibodies could recognize human IFN-beta in comparison with antibodies from serum of a healthy individual. A mouse antiserum to human IFN-beta recognized rsgp41 (recombinant soluble gp41 Env amino acid 539-684), while the normal mouse serum (pre-immune serum) did not bind to rspg41. These results indicate that a common immunological epitope exists between human IFN-beta and HIV-1 gp41. The sequence-similarity suggests that this common immunological epitope may be located in the region aa128-134 of human IFN-beta and the immunosuppressive domain (aa583-599) of HIV-1 gp41. The increased levels of antibodies against interferon-beta in HIV-1 positive individuals may be explained by a common immunological epitope on human IFN-beta and HIV-1 gp41.

The immunosuppressive peptide of HIV-1 gp41 like human type I interferons up-regulates MHC class I expression on H9 and U937 cells

Based on our findings that the immunosuppressive peptide (ISP, amino acids (aa) 583-599) of human immunodeficiency virus type 1 (HIV-1) gp41 shows sequence-similarity with human type I interferons (IFN-alpha and IFN-beta) and HIV-1 soluble gp41 (sgp41, aa 539-684) enhanced cell surface expression of major histocompatibility complex (MHC) class I molecule on human H9 (T cells), Raji (B cells) and U937 (monocytic cells) cells, we examined the effect of HIV-1 immunosuppressive peptide on the surface expression of MHC class I molecules on H9 and U937 cells. Flow cytometry analysis demonstrated that ISP-BSA (conjugate) could enhance MHC class I expression by about 40% on H9 cells and by about 45% on U937 cells, while monomer ISP (not conjugated) and EDCI-treated carrier protein (BSA-EDCI) did not increase the expression. By comparison, human type I interferons, IFN-alpha and IFN-beta, showed similar effects (enhanced the expression by about 40-60%) to ISP-BSA on the MHC class I expression on H9 and U937 cells. The results suggest that HIV-1 gp41 in a polymerized form by its immunosuppressive domain upregulates human MHC class I expression. The basis for this similar effect of HIV-1 gp41 and IFN-alpha and -beta, i.e. upregulation of MHC class I molecule expression, may be based on the sequence-similarity between these otherwise different molecules.

A recombinant single-chain human class II MHC molecule (HLA-DR1) as a covalently linked heterotrimer of alpha chain, beta chain, and antigenic peptide, with immunogenicity in vitro and reduced affinity for bacterial superantigens

Major histocompatibility complex (MHC) class II molecules bind to numerous peptides and display these on the cell surface for T cell recognition. In a given immune response, receptors on T cells recognize antigenic peptides that are a minor population of MHC class II-bound peptides. To control which peptides are presented to T cells, it may be desirable to use recombinant MHC molecules with covalently bound antigenic peptides. To study T cell responses to such homogeneous peptide-MHC complexes, we engineered an HLA-DR1 cDNA coding for influenza hemagglutinin, influenza matrix, or HIV p24 gag peptides covalently attached via a peptide spacer to the N terminus of the DR1 beta chain. Co-transfection with DR alpha cDNA into mouse L cells resulted in surface expression of HLA-DR1 molecules that reacted with monoclonal antibodies (mAb) specific for correctly folded HLA-DR epitopes. This suggested that the spacer and peptide did not alter expression or folding of the molecule. We then engineered an additional peptide spacer between the C terminus of a truncated beta chain (without transmembrane or cytoplasmic domains) and the N terminus of full-length DR alpha chain. Transfection of this cDNA into mouse L cells resulted in surface expression of the entire covalently linked heterotrimer of peptide, beta chain, and alpha chain with the expected molecular mass of approximately 66 kDa. These single-chain HLA-DR1 molecules reacted with mAb specific for correctly folded HLA-DR epitopes, and identified one mAb with [MHC + peptide] specificity. Affinity-purified soluble secreted single-chain molecules with truncated alpha chain moved in electrophoresis as compact class II MHC dimers. Cell surface two-chain or single-chain HLA-DR1 molecules with a covalent HA peptide stimulated HLA-DR1-restricted HA-specific T cells. They were immunogenic in vitro for peripheral blood mononuclear cells. The two-chain and single-chain HLA-DR1 molecules with covalent HA peptide had reduced binding for the bacterial superantigens staphylococcal enterotoxin A and B and almost no binding for toxic shock syndrome toxin-1. The unique properties of these engineered HLA-DR1 molecules may facilitate our understanding of the complex nature of antigen recognition and aid in the development of novel vaccines with reduced superantigen binding.

Increased levels of antibodies against interferon-alpha in HIV-1 positive individuals may be explained by a common immunological epitope on the human interferon-alpha and HIV-1 gp41

Based on the similar effects of HIV-1 gp41 and human type I interferons on inhibition of lymphocyte proliferation and modulation of MHC class I and II molecule expression, we compared amino acid sequences of human interferons with HIV-1 gp41 and found sequence-similarity existing between gp41 and IFN-alpha. Anti-gp41-antibodies affinity-purified from sera of HIV-1-infected individuals (stage A) using rsgp41-Sepharose column could recognize human IFN-alpha in ELISA, but no antibody against IFN-alpha was detected if immunoglobulins were prepared in the same way from pooled HIV-negative serum. Besides, a sheep polyclonal anti-human IFN-alpha antibody bound weakly to recombinant soluble gp41 (rsgp41) of HIV-1IIIB. These results indicate that gp41 may share an immunological epitope with IFN-alpha. We examined 40 sera from healthy and asymptomatic HIV-1-infected individuals and AIDS-patients for IFN-alpha antibody levels by ELISA. The levels of anti-IFN-alpha antibody in sera from asymptomatic HIV-infected individuals (stage A, n = 6) was increased by about 150% in comparison with HIV-negative, but the antibody levels were obviously reduced in the case of symptomatic HIV-infected individuals (stage B, n = 7) and AIDS-patients (stage C, n = 7) in comparison with asymptomatic HIV-infected individuals (stage A). We suppose that the increased IFN-alpha-antibody level in HIV-1-infected individuals (stage A) may be due to this common immunological epitope and cross-reaction of antibodies to HIV-1 gp41 with IFN-alpha.

Role of complement in HIV infection

In human plasma, HIV activates the complement system, even in the absence of specific antibodies. Complement activation would, however, be harmful to the virus if the reactions were allowed to go to completion, since their final outcome would be virolysis. This is avoided by complement regulatory molecules, which either are included in the virus membrane upon budding from the infected cells (e.g. DAF/CD55) or are secondarily attached to HIV envelope glycoproteins as in the case of factor H. By using this strategy of interaction with complement components, HIV takes advantage of human complement activation for enhancement of infectivity, for follicular localization, and for broadening its target cell range at the same time that it displays an intrinsic resistance against the lytic action of human complement. This intrinsic resistance to complement-mediated virolysis can be overcome by monoclonal antibodies inhibiting recruitment of human factor H to the virus surface, suggesting a new therapeutic principle.