Evolution and immunopathology of chikungunya virus informs therapeutic development

Abstract

Chikungunya virus (CHIKV), a mosquito-borne alphavirus, is an emerging global threat identified in more than 60 countries across continents. The risk of CHIKV transmission is rising due to increased global interactions, year-round presence of mosquito vectors, and the ability of CHIKV to produce high host viral loads and undergo mutation. Although CHIKV disease is rarely fatal, it can progress to a chronic stage, during which patients experience severe debilitating arthritis that can last from several weeks to months or years. At present, there are no licensed vaccines or antiviral drugs for CHIKV disease, and treatment is primarily symptomatic. This Review provides an overview of CHIKV pathogenesis and explores the available therapeutic options and the most recent advances in novel therapeutic strategies against CHIKV infections.

Keywords: Antiviral compounds; Chikungunya pathogenesis; Chikungunya virus; Immunomodulatory drugs; Monoclonal antibodies; Vaccines.

Fig. 1.

CHIKV life cycle. (1) CHIKV enters the host cells through receptor-mediated endocytosis. The viral protein E2 interacts with specific host surface receptors. A number of receptors [for example, prohibitin (Wintachai et al., 2012), MXRA8 (Zhang et al., 2018), PS receptors (Moller-Tank et al., 2013) and GAG (Silva et al., 2014)] have been implicated in this process. (2) Once in the endosome, the acidic pH triggers conformational changes in the viral envelope, exposing the E1 peptide and leading to its fusion with the endosomal membrane (Voss et al., 2010). (3) This allows for the cytoplasmic delivery of the nucleocapsid and the release of the viral RNA genome. (4) The viral genome is translated to the non-structural polyprotein nsP1- nsP2-nsP3-nsP4. (5) The viral protease nsP2 then cleaves the polyprotein into the individual nsPs – nsP1, nsP2, nsP3 and nsP4 – that then form the viral replicase complex. (6,7) The viral replicase is responsible for the synthesis of the negative-strand RNA that will be a template for both new positive-strand RNA and the sub-genomic RNA (26S RNA) (van der Heijden and Bol, 2002). (8) The 26S RNA drives the expression of the structural polyprotein C-pE2-6K-E1 in the endoplasmic reticulum. (9,10) The capsid protein (C) dissociates from the polyprotein by self-cleavage activity and binds to the newly synthesised viral RNA, forming the nucleocapsid core in the cytoplasm. (11) In the meantime, E2 and E1 associate in the Golgi apparatus and are exported to the cell membrane, where pE2 is cleaved by the host protease furin into E2 and E3. E3, which stabilises the E2/E1 trimer, then dissociates from the trimer when it reaches the cell membrane. (12) The already-formed nucleocapsid migrates to the host cell membrane region rich in E2/E1 trimers that bind the virion membrane. (13) The mature virions are released by the budding process from the infected cells (Yap et al., 2017). CHIKV, chikungunya virus; GAG, glycosaminoglycan; MXRA8, matrix remodelling-associated protein 8; nsP, non-structural protein; PS receptor, phosphatidylserine-mediated entry-enhancing receptor.

Fig. 2.

CHIKV infection activates host immune responses, leading to joint/muscle inflammation. CHIKV is detected by TLR3, TLR7 and TLR8, as well as by RIG-I-like receptors, which activate transcription factors, NF-κB and IRFs. IRFs stimulate a strong antiviral type I IFN response through the transcription of IFN-stimulated genes that encode antiviral proteins, such as viperin, IFN-α/β and OAS (Teng et al., 2012; Priya et al., 2014; Onomoto et al., 2021). NF-κB activates a pro-inflammatory response by stimulating the transcription of many pro-inflammatory cytokines and chemokines, including TNF-α and CCL2 (Teng et al., 2015; Onomoto et al., 2021). The chemokine CCL2 is responsible for the recruitment of monocytes/macrophages to the site of infection, while TNF-α has been linked to the recruitment of cytolytic lymphocytes, including NK cells and CD8⁺ T cells that secrete the pro-inflammatory granule GZMA, which has demonstrated a prominent role in driving arthritic inflammation. Studies have also shown that antibody-producing B cells are involved in CHIKV clearance and control. CD4⁺ T cells were shown to be activated during the chronic phase of CHIKV infection and play a role in pathogenesis of CHIKV-induced joint inflammation. CD8⁺ T cells, neutrophils and monocytes/macrophages can also accumulate in the joints, with monocytes and macrophages even differentiating into osteoclasts that can lead to damage in the joint. CCL2, chemokine ligand 2; CHIKV, chikungunya virus; IFN, interferon; IRF, interferon regulatory factor; GZMA, granzyme A; NF-κB, nuclear factor kappa-light-chain-enhancer of activated B cells; NF-κB, nuclear factor kappa B; NK, natural killer; OAS, 2'-5'-oligoadenylate synthetase 1; RIG-I, retinoic acid-inducible gene I; TLR, Toll-like receptor; TNF-α, tumour necrosis factor alpha.

Fig. 3.

CHIKV treatments targeting the viral life cycle, host defence mechanisms and host immunopathology. Antivirals that block viral entry into the host cell include the natural compounds epigallocatechin gallate (Weber et al., 2015), flavaglines (Wintachai et al., 2015) and curcumin (Mounce et al., 2017). In vitro, chloroquine was shown to increase the endosomal pH, which then prevents the fusion of CHIKV E1 glycoprotein with the endosomal membrane (Khan et al., 2010). Broad-spectrum antivirals, including ribavirin and 6-azauridine, inhibit viral replication by introducing mutations in the viral genome, whereas favipravir targets RNA polymerase and prevents viral replication (Briolant et al., 2004). The viral nsPs are also a potential target for the development of antiviral molecules, and several compounds have been identified to target different nsPs, including harringtonine (Bassetto et al., 2013; Das et al., 2016). In response to CHIKV infection, the host elicits a strong antiviral type I IFN; therefore, recombinant IFN-α has shown the ability to inhibit CHIKV replication in vitro (Briolant et al., 2004). Furthermore, activating the RIG-I receptor with 5′ triphosphorylated RNA can augment this host response (Olagnier et al., 2014). A prolonged pro-inflammatory host response to CHIKV infection can lead to the chronic disease; therefore, some treatments are aimed at dampening this response. Inhibiting the chemokine CCL2 with bindarit ameliorated the inflammation in joints and skeletal muscles in mice (Rulli et al., 2011). TNF-α inhibitors, such as etanercept and adamimumab, were shown to reduce arthritic symptoms in patients (Blettery et al., 2016). CD4⁺ T cells also have a primary role in the pathogenesis of CHIKV-induced joint inflammation (Teo et al., 2017). Fingolimod efficiently suppressed CHIKV-induced joint pathology in virus-infected mice by blocking T-cell migration from the lymph nodes to the joints (Teo et al., 2017), and abatacept, which inhibits CD4⁺ T-cell priming in combination with neutralising anti-CHIKV monoclonal antibody, reduced T-cell accumulation in the joints (Miner et al., 2017). This resulted in ameliorated joint inflammation and decrease in proinflammatory cytokine secretion (Miner et al., 2017). The pro-inflammatory granule GZMA is released by natural killer cells and CD8⁺ T cells in response to CHIKV infection, and the GZMA inhibitor, Serpinb6b, was shown to reduce joint inflammation (Wilson et al., 2017). DMARDs, including hydroxychloroquine, sulfasalazine and methotrexate, have been trialled for chronic CHIKV arthritis (Ganu and Ganu, 2011; Martí-Carvajal et al., 2017; Ravindran and Alias, 2017). However, owing to contradictory results, it remains unclear whether the use of specific DMARDs is effective in treating chronic CHIKV arthritis. CHIKV, chikungunya virus; DMARD, disease-modifying antirheumatic drug; GZMA, granzyme A; IFN, interferon; IRF, interferon regulatory factor; NF-κB, nuclear factor kappa B; nsP, non-structural protein; RIG-I, retinoic acid-inducible gene I; TNF-α, tumour necrosis factor alpha.

Fig. 4.

Several platforms used for the development of CHIKV vaccines that have entered human clinical trial testing. (A) LAV vaccines are derived from CHIKV, with structural changes introduced to reduce its virulence but retain its immunogenicity. The CHIKV LAV vaccines that have reached human clinical trials include TSI-GSD (The Salk Institute-Government Services Division), which showed promising results in phase I and II trials, but, owing to limited funding and uncertainty about safety, the development of this vaccine was terminated (Edelman et al., 2000); the Δ5nsP3 vaccine (VLA1553), which recently entered phase III clinical trial (NCT04546724); and IRES-CHIKV, which is in its late preclinical stages, in which immunogenicity and efficacy have been tested in mice and non-human primates (Plante et al., 2011; 2015). (B) The use of whole inactivated virus via chemical treatment, with formalin, was one of the first strategies for the development of a vaccine for CHIKV. The formalin-inactivated CHIKV vaccine entered clinical phase I (Harrison et al., 1971); however, despite the good safety, tolerability and immunogenicity profiles, the vaccine programme was discontinued. (C) VLP vaccines closely resemble the wild-type virus by containing self-assembled structural proteins but no viral genetic material. A CHIKV VLP vaccine candidate, VRC-CHKVLP059-00-VP, has now advanced into clinical phase II evaluation (NCT02562482) (Akahata et al., 2010). (D) Chimeric or recombinant viral-vectored vaccines are obtained by incorporating genetic elements of CHIKV into a vector virus genome. The recombinant viral-vectored vaccine MV-CHIKV, which uses attenuated measles virus as a vector, is currently in clinical phase II (NCT02861586), and ChAdOx1 Chik vaccine, known as the chimpanzee adenovirus Oxford 1 as it uses the Simian adenovirus as a vector, is in phase I of a clinical trial (NCT03590392). (E) mRNA vaccines are the newest strategy for the development of CHIKV vaccine candidates, based on the delivery of engineered mRNAs that can instruct host cells to express viral antigens, mimicking a viral infection that could elicit an immune response and lead to the generation of antibodies. There is currently one CHIKV mRNA vaccine, VLA-181388, in phase I clinical trial (NCT03325075) (Goyal et al., 2018). CHIKV, chikungunya virus; LAV, live-attenuated virus; VLP, virus-like particle.