Clade I (Congo Basin Clade)

Clade I of the MPXV, primarily found in Central Africa, particularly the DRC, is associated with more severe disease and higher mortality rates, ranging from 4% to 11% in some studies [14]. This Clade has been recognized for a longer period and has historically caused the majority of endemic cases in Africa. Recent findings indicate that Clade I exhibits more efficient human-to-human transmission compared to Clade II, particularly in settings where close contact occurs, such as healthcare or familial environments. The emergence of new lineages within Clade I, such as Clade Ib, has raised concerns regarding their potential for increased transmissibility and virulence. Genomic analyses have shown that Clade Ib is distinct from previously circulating strains and has been linked to sexual transmission routes, indicating a shift in transmission dynamics. Surveillance data from 2023 to early 2024 revealed that a significant proportion of cases involved younger populations, with many being sex workers, suggesting that behavioral factors may play a role in the spread of this Clade [15].

Clade II (West African Clade)

In contrast, Clade II, predominantly found in West Africa, encompasses subclades such as IIa and IIb, with cases reported in countries like Nigeria and Cameroon. This Clade is generally associated with milder disease and lower mortality rates, typically around 1% to 3%. Although it is less efficient in human-to-human transmission compared to Clade I, outbreaks have still occurred, particularly during the global epidemic that began in 2022. Clade IIb gained significant attention due to its rapid spread outside endemic regions during the recent global outbreak, with genomic analyses revealing mutations indicative of adaptation to human hosts. The B.1 lineage of Clade IIb was responsible for a substantial number of cases globally, demonstrating a shift toward increased transmissibility through sexual contact [16].

Interaction with infected animals

Direct contact with infected animals remains a major risk factor in mpox transmission. Recent findings highlight the fire-footed rope squirrel (Funisciurus pyrropus) as a potential reservoir, capable of asymptomatic viral shedding [19]. Evidence from a 2023 mangabey outbreak supports cross-species transmission, while pouched rats and other rodents may also contribute to a broader reservoir network. Transmission occurs through direct handling, exposure to bodily fluids, bites, or indirect contact, such as consuming undercooked bushmeat [20]. A meta-analysis reports a 5.61-fold increased infection risk among individuals exposed to infected animals [21]. Activities like hunting, trapping, and skinning are particularly hazardous. However, identifying true reservoir hosts remains challenging due to limited surveillance and the complex ecology of mpox in endemic regions.

HIV infection

Mpox infection elicits strong immune activation characterized by an increase in cluster of differentiation 38 (CD38)⁺, HLA-DR⁺ CD8⁺ T cells, which are essential for viral control but may also contribute to tissue damage [22]. Alterations in CD4⁺ T cell subsets, particularly rise in effector and transitional memory cells, reflect adaptive immune engagement. Elevated plasma levels of regulated upon activation, normal T cell expressed and secreted (RANTES), chemokine C-C motif ligand 3 (CCL3), C-X-C motif chemokine ligand 10 (CXCL10), and interleukin-2 receptor α (IL-2Rα) indicate a robust inflammatory response, while interleukin-6 (IL-6) shows variable expression. B cell responses include a shift from IgA to IgG isotypes in plasmablasts, signaling adaptive humoral immunity [23]. People living with HIV (PLHIV), especially those with advanced immunosuppression, are at significantly higher risk for mpox infection and severe disease outcomes [24]. A 2024 meta-analysis found that PLHIV are 4.46 times more likely to contract mpox than HIV-negative individuals, with the risk increasing markedly in those with CD4 counts below 200 cells/mm³ [21]. These individuals often experience severe clinical manifestations, including extensive lesions and systemic complications, leading to elevated mortality. Hospitalization risk is also heightened, with a 56.6% increase compared to HIV-negative patients [25]. During the 2022–2023 outbreak, PLHIV accounted for 38–50% of global cases, underscoring the public health burden. Despite many having well-controlled HIV, those with advanced disease faced worse outcomes. Immunologically, impaired cellular responses in advanced HIV hinder viral control, contributing to mpox severity [26].

Other sexually transmitted infections

Recent systematic reviews and meta-analyses have confirmed that pre-existing sexually transmitted infections (STIs) significantly increase the risk of acquiring mpox, particularly during recent outbreaks marked by sexual transmission among MSM [27]. A pooled OR of 1.76 (95% CI: 1.42–2.91) demonstrates a 76% increased risk of mpox in individuals with prior STIs such as syphilis, gonorrhea, chlamydia, hepatitis B/C, and varicella-zoster virus. Moderate heterogeneity (I² ≈ 40%) is reported, with results remaining statistically significant (P < 0.0001) [28]. Additionally, comorbidities, including STIs, elevate mpox risk (OR = 1.58, 95% CI: 1.31–1.91), and multiple sexual partners further compound susceptibility (OR ≈ 1.6–1.8) [21]. Biological plausibility supports this association, as STI-related mucosal disruption and immune suppression increase mpox susceptibility. Mpox lesions also facilitate STI transmission, creating a bidirectional risk [28]. The 2022 outbreak strains (Clade IIb, lineage B.1) show rapid evolution driven by apolipoprotein B mRNA-editing enzyme catalytic polypeptide-like 3 (APOBEC3)-mediated mutations (e.g., GA>AA, TC>TT), with 66–86 nucleotide and 28–39 amino acid substitutions, particularly in immune-related genes such as B21R, F13L, and A18R [29]. Mpox employs multiple immune evasion mechanisms, including nuclear factor kappa-light-chain-enhancer of activated B cells (NF-κB) pathway inhibition via ankyrin-like and B cell lymphoma 2 (BCL-2)-like proteins, T cell suppression by B21R, and mucosal barrier disruption through genital/rectal lesions [30]. Co-infections, especially with HIV, intensify disease severity through immune compromise (e.g., CD4⁺ T cell depletion), increased pathogen load, and delayed viral clearance, factors that may elevate the risk of tecovirimat resistance.

Sexual behavior

Recent data reinforce sexual behavior as a key driver of mpox transmission in non-endemic regions. A case-control study in California (2022–2023) revealed a significantly elevated infection risk from sexual contact with asymptomatic or suspected mpox partners (OR = 7.7) [31]. Risk increased with the number of partners: individuals with ≥ 4 partners had nearly fourfold higher odds of infection (OR = 3.8) [21]. Site-specific sexual practices correlated with lesion location, linking insertive and receptive anal sex to penile and anorectal lesions, respectively [31]. A 2023 Chinese study found that safer sexual behaviors, such as avoiding group or commercial sex and using condoms, significantly reduced mpox risk. Meta-analyses further support these findings, identifying multiple partners (OR = 1.61), unprotected sex (OR = 1.53), HIV (OR = 4.46), and other STIs (OR = 1.76) as key risk factors [21]. These insights highlight the critical role of sexual networks and behavior in the spread and the importance of behavioral interventions in outbreak control.

Routes and mechanisms

MPXV transmission involves multiple routes, with recent outbreaks emphasizing human-to-human spread, particularly through sexual contact. Zoonotic transmission persists in endemic regions via rodent reservoirs, such as Funisciurus spp. and Cricetomys gambianus, with viral shedding through saliva, lesions, and excreta [32]. Bushmeat handling and consumption remain relevant risk factors. Human-to-human transmission is now primarily driven by sexual exposure, with culturable virus detected in semen up to 21 days post-symptom onset and anogenital lesions present in 60–70% of cases, facilitating mucosal entry [33]. While respiratory droplet transmission occurs during prolonged close contact, no evidence supports aerosol spread outside healthcare settings. Fomite transmission is substantiated by MPXV persistence on surfaces for over 15 days. Vertical transmission is rare but confirmed, with placental infection linked to high fetal mortality [34]. At the cellular level, MPXV entry is mediated by mature virion (MV) proteins (A26, A27, H3, D8) binding to glycosaminoglycans and EV proteins (B5, A33), promoting actin-driven intercellular spread [35]. Fusion is pH-independent, involving 11–12 transmembrane proteins and host actin remodeling. Early immune evasion involves the suppression of NF-κB by ankyrin-repeat proteins (J3L, D1L) and the inhibition of apoptosis via BCL-2-like protein (A47R) [36]. Following mucosal entry, the virus disseminates via lymphatic spread, leading to viremia and infection of distant organs, including skin, liver, testes, and placenta. Genomic adaptations, notably APOBEC3-mediated mutations and gene duplications in immunomodulatory loci, enhance host adaptation and immune evasion [37].

MPXV replication and cytoplasmic life cycle

The replication of MPXV follows a complex cytoplasmic life cycle distinct from most DNA viruses [38]. MPXV initiates infection by binding to host cell surface molecules such as glycosaminoglycans (heparan sulfate, chondroitin sulfate) and integrins via viral surface proteins like hemagglutinin [39]. Entry occurs either through direct membrane fusion or clathrin-mediated endocytosis. Upon entry, the viral core is uncoated, and the genome is released directly into the cytoplasm, bypassing the nucleus [40]. MPXV gene expression proceeds in a tightly regulated cascade where early genes are expressed immediately post-entry and encode proteins for immune evasion and replication initiation; intermediate genes trigger viral DNA replication; late genes encode structural proteins required for virion assembly. Replication occurs in cytoplasmic viral factories called Guarnieri bodies [41]. Viral DNA replication produces concatemeric intermediates that are processed into individual genomes. These are packaged into IMVs, which mature into stable intracellular MVs. Some MVs acquire additional membranes from the Golgi or endosomal system, forming EEVs, which enhance cell-to-cell spread and immune evasion. Viral egress involves lysis for MVs and exocytosis for EEVs. MPXV virions carry multiple immunomodulatory proteins that inhibit host inflammatory responses, apoptosis, and complement activation [42].

Viral evasion of host immune responses

MPXV has evolved complex mechanisms to evade host immune responses, facilitating persistent infection and effective transmission (Figure 2). These strategies target innate and adaptive immune systems, ensuring viral survival despite host defenses. In the innate immune response, MPXV suppresses type I IFNs, critical for antiviral defense, by inhibiting pattern recognition receptors (PRRs) such as retinoic acid-inducible gene I (RIG-I), melanoma differentiation-associated protein 5 (MDA5), and Toll-like receptors (TLRs), which would normally activate antiviral transcription factors like IFN regulatory factor 3 (IRF3) and NF-κB. Viral proteins like E3L and K3L block key antiviral pathways, including the protein kinase R (PKR) and 2',5'-oligoadenylate synthetase (OAS)/ribonuclease L (RNase L) systems, preventing antiviral protein synthesis and RNA degradation [43]. MPXV also evades type II IFN (IFN-γ) signaling, impeding the Janus kinase (JAK)/signal transducers and activators of transcription (STAT) pathway [44]. The virus further inhibits the complement system, preventing the formation of the membrane attack complex (MAC) and protecting infected cells from lysis [45]. In the adaptive immune response, MPXV modulates T cell activity by interfering with MHC expression and TCR signaling, thereby reducing CD8⁺ T cell activation and differentiation [46]. It also blocks apoptosis in infected cells, allowing prolonged viral replication. Furthermore, MPXV evades neutralizing antibodies by utilizing cell-associated viremia, where infected monocytes shield the virus from antibodies, and by expressing Fc-binding proteins that prevent antibody-dependent cellular cytotoxicity [47]. MPXV also alters the cytokine environment by suppressing pro-inflammatory cytokines like tumor necrosis factor alpha (TNF-α) and IL-1β and sequestering chemokines such as CCL5 and CXCL10, impairing immune cell recruitment [48]. These immune evasion mechanisms contribute to MPXV’s ability to persist in the host, evade immune surveillance, and enhance transmission [13].

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Figure 2. Viral immune evasion cascades in MPXV infection. MPXV: monkeypox virus; MDA5: melanoma differentiation-associated protein 5; RIG-I: retinoic acid-inducible gene I; TLRs: Toll-like receptors; IRF3: interferon regulatory factor 3; NF-κB: nuclear factor kappa-light-chain-enhancer of activated B cells; TNF-α: tumor necrosis factor alpha; IL-1β: interleukin-1 beta; CCL5: chemokine C-C motif ligand 5; CXCL10: C-X-C motif chemokine ligand 10; PKR: protein kinase R; OAS: 2',5'-oligoadenylate synthetase; RNase L: ribonuclease L; MHC: major histocompatibility complex; CD8: cluster of differentiation 8; JAK: Janus kinase; STAT: signal transducers and activators of transcription

A critical mechanism involves inhibition of the complement cascade via the viral MOPICE protein, a homolog of the VACV complement control protein. MOPICE mimics host regulators, binds C3b/C4b, and acts as a decay-accelerating factor to block convertase formation, thereby reducing MAC assembly and inflammatory signaling [49]. Additionally, MPXV suppresses IFN signaling through the B16 protein, which sequesters IFN-α/β extracellularly, preventing JAK1/tyrosine kinase 2 (TYK2) activation and downstream STAT1/2-mediated transcription of IFN-stimulated genes (ISGs) [50]. This significantly impairs early antiviral responses. In the adaptive immune arm, MPXV interferes with MHC-I antigen presentation through B10 and N3R proteins. B10 retains MHC-I in the endoplasmic reticulum, while N3R disrupts peptide loading, both resulting in impaired cytotoxic T lymphocyte recognition [51]. The virus also dampens inflammatory pathways by targeting NF-κB activation. Ankyrin repeat-containing and BCL-2-like proteins bind host IKK complexes and TNF receptor-associated factor 6 (TRAF6) adaptors, respectively, inhibiting nuclear translocation of NF-κB and reducing cytokine production [52]. Furthermore, the B21R glycoprotein directly suppresses T cell activation by modulating TCR signaling, lowering IL-2 and IFN-γ expression, and impairing CD4⁺/CD8⁺ T cell function [53]. Together, these mechanisms highlight MPXV’s ability to evade immune detection at multiple molecular checkpoints.

Poxin-mediated immune evasion in mpox impairs cGAS-STING signaling

MPXV encodes a specialized nuclease, poxin, which plays a critical role in evading host innate immunity by targeting the cyclic GMP-AMP synthase-stimulator of IFN genes (cGAS-STING) signaling axis (Figure 3). Under normal conditions, cGAS detects cytosolic double-stranded DNA and catalyzes the synthesis of cyclic GMP-AMP (cGAMP), a second messenger that activates STING [62]. STING engagement triggers the phosphorylation of TANK-binding kinase 1 (TBK1), IRF3, and NF-κB, ultimately driving the expression of IFNs and pro-inflammatory cytokines such as IL-6 and TNF-α [63]. Poxin acts by specifically hydrolyzing 2',3'-cGAMP, thereby abolishing STING activation and downstream signal transduction [64]. Structural and enzymatic analyses have identified key residues His17, Tyr138, and Lys142 as essential for catalysis, forming a conserved triad within the active site. Unlike conventional nucleases, poxin does not require divalent metal ions for activity, instead utilizing a metal-independent mechanism likely based on acid-base catalysis. Moreover, the enzyme exhibits stringent substrate specificity, selectively degrading 2',3'-cGAMP while sparing bacterial cyclic dinucleotides such as 3',3'-cGAMP and c-di-GMP [63]. Through this mechanism, MPXV poxin effectively suppresses the host’s early antiviral response, impairing the activation of TBK1 and IRF3 and limiting the production of IFNs and inflammatory mediators [65]. This enables the virus to evade detection during the initial phase of infection and facilitates viral replication and spread, particularly in epithelial barriers where nucleic acid sensing is pronounced. Poxin thus represents a highly refined viral adaptation for immune modulation, underscoring its significance in MPXV pathogenesis and its potential as a therapeutic target.

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Figure 3. Modulation of the cGAS-STING antiviral response by MPXV. cGAS-STING: cyclic GMP-AMP synthase-stimulator of interferon genes; MPXV: monkeypox virus; cGAMP: cyclic GMP-AMP; IRF3: interferon regulatory factor 3; TBK1: TANK-binding kinase 1; IFNs: interferons; TNF-α: tumor necrosis factor alpha; IL-6: interleukin-6; P: phosphorylation

Viral persistence and systemic spread

Once MPXV enters the human body, it replicates initially at the entry site, typically leading to local infection [13]. This is commonly manifested as the characteristic skin lesions, an important clinical indicator of the virus. Early systemic symptoms often include fever, headache, malaise, and lymphadenopathy, which reflect the virus’s systemic spread [72]. The virus then disseminates via the lymphatic system to regional lymph nodes, where it further amplifies [12]. It subsequently enters the bloodstream, leading to viremia, which allows the virus to spread to various organs, including the skin, lungs, liver, spleen, and kidneys [73]. The presence of systemic involvement and widespread rash is a key hallmark of MPXV infection. In certain cases, MPXV can invade the CNS, leading to severe neurological complications such as encephalitis [74]. This may result from direct viral infection of the brain and spinal cord tissues, although such instances are relatively rare. Additionally, MPXV can target immune cells, particularly lymphoid tissues, and modulate immune responses, facilitating viral persistence and replication [11]. This immune evasion is critical for the virus as it allows the establishment of an infection that can evade the host’s immune defenses for extended periods. One notable aspect of MPXV infection is its potential for viral persistence. After the acute phase of infection, the virus can remain in certain tissues for extended periods, possibly leading to latent or subclinical infections. This persistence may result in individuals continuing to shed the virus even after clinical symptoms have subsided, which poses a significant risk for ongoing transmission [75]. These asymptomatic or subclinical carriers may harbor the virus in tissues such as lymphoid tissues, skin, and possibly even the genital tract, maintaining a reservoir of infection in the population [11].

MPXV employs a variety of immune evasion mechanisms to facilitate persistence and systemic spread. These mechanisms include the inhibition of IFN production, modulation of T cell function, suppression of apoptosis, and other strategies to dampen antiviral immune responses [44]. This allows MPXV to circumvent the host’s immune surveillance, replicate efficiently, and potentially lead to chronic or persistent infection, prolonging transmission risk. The hallmark clinical feature of MPXV infection is the development of skin lesions, which evolve from macules to papules, vesicles, pustules, and eventually scabs [76]. These lesions typically appear all over the body, particularly on the face, palms, soles of the feet, and genital areas. The progression of these lesions provides essential diagnostic information for identifying the virus. While most cases of MPXV infection resolve with supportive care, more severe cases can occur, particularly in immunocompromised individuals, leading to complications such as secondary bacterial infections or systemic organ failure. In rare instances, these severe infections can be fatal. The risk of severe disease highlights the importance of early diagnosis, clinical management, and preventive measures, especially in at-risk populations.