Antiviral innate immunity and adaptive immune responses against monkeypox and viral immune evasion

Abstract

The monkeypox virus (MPXV) belongs to the Poxviridae family and has emerged as a global problem for public health since May 2022. Both innate and adaptive immunity are crucial for preventing MPXV infections. Understanding the interaction between the innate and adaptive immune systems and MPXV could be critical in developing novel treatments. This review presents an update on the host antiviral innate and adaptive immune responses against four zoonosis poxvirus, including mouse kidney parvovirus, cowpox virus, variola virus, and vaccinia virus. The effect of APOBEC3 variants on viral immune escape and the underlying mechanisms were also explored. The possible mechanisms of the effects of human immunodeficiency virus and monkeypox co-infection in the clinical setting were also analysed in the hope of providing a basis for clinical drug development and treatment.

Key Words: Immune evasion; Adaptive immune; Innate immune; Monkeypox virus; Poxviridae family

Core Tip: The monkeypox virus has emerged as a global public health concern since 2022. It provides a comprehensive update on the host’s antiviral innate and adaptive immune responses against monkeypox virus and related orthopox viruses. The article further explores the complex mechanisms of viral immune evasion, including strategies such as APOBEC3-driven mutations that may influence viral evolution and immune recognition. Understanding these intricate host-pathogen interactions is critical for developing novel treatments and next-generation vaccines.

INTRODUCTION

Monkeypox (MPX) is a zoonotic infectious disease caused by the MPXV[1]. Once a neglected tropical disease with limited geographic spread, MPX has recently emerged as a global public health concern. MPXV was first isolated and observed from cynomolgus monkeys in Copenhagen in 1958[2]. The Democratic Republic of Congo saw the first reports of zoonotic transmission of MPXV from animal to human in 1997[3]. For decades thereafter, cases were largely confined to Central and West Africa, with sporadic reports in Liberia, Nigeria, and Sierra Leone[1,4,5]. A significant milestone occurred in 2003 with the first outbreak outside Africa in the United States, demonstrating the virus’s potential for geographic spread[6]. Despite the documented presence of limited human-to-human transmission in the United Kingdom in 2018, the epidemiological pattern of the virus has consistently been characterised by low prevalence and episodic spillover events - until 2022.

The symptoms of MPXV infection are similar to those of its close cousin, smallpox, but are less severe[7]. The incubation period of MPXV is 1-2 weeks[8]. MPX’s most common clinical features are fever, fatigue, headache, swollen lymph nodes (in the neck, armpits, or groin), and respiratory symptoms. The duration of these symptoms is typically 2-4 weeks[9]. In some clinical reports, MPXV can induce some complications, such as encephalitis, bronchopneumonia, secondary infection of the integument, and sepsis, which can occur[10]. However, it often causes more severe symptoms and even death in people with compromised immunity, such as human immunodeficiency virus patients and children[11,12]. The MPX mortality rate is generally 1%-10%. The global context of MPX was radically altered in 2022 by the emergence of widespread, simultaneous outbreaks across numerous non-endemic countries. This event marked a decisive shift from a pattern of sporadic, localized zoonotic spillovers to sustained human-to-human transmission networks on an international scale. The latest genomic analysis of the virus reveals characteristic APOBEC3 cytidine deaminase activity, resulting in a significantly elevated C-to-T mutation rate. Whilst these mutations do not alter the virus's fundamental phylogenetic classification, they may influence immune recognition and vaccine efficacy. This may account for the 2022 outbreak and the emergence of human-to-human transmission.

MPXV became a pandemic worldwide after an imported case from Nigeria was reported in the United Kingdom on May 23, 20238. Subsequently, Belgium became the first country to declare a mandatory 21-day quarantine against MPXV[13]. In July 2022, the World Health Organization declared the ongoing MPX outbreak a Public Health Emergency of International Concern (PHEIC)[14]. MPX cases have been reported in 111 countries, of which 104 are first-reported cases (Available from: https://ourworldindata.org/monkeypox, visit on 31 January, 2026), which has nothing to do with travel to endemic countries of many MPX cases. These countries are mainly Western Europe, Central Europe, North America, and Australia. At present, the cumulative total of 179 cases, including 144 deaths (Available from: https://www.cdc.gov/poxvirus/monkeypox/response/2022/world-map.html, visit on 31 January, 2026).

SPECIES, STRUCTURE, AND STRAIN OF MPXV

Species

The Poxviridae family can be separated into two subfamilies depending on their animal hosts: Chordopoxvirinae and Entomopoxvirinae. The former subfamily is known to infect vertebrates, and it is differentiated into 18 genera, including Avipoxvirus, Capripoxvirus, Cervidpoxvirus, Leporipoxvirus, Molluscipoxvirus, Orthopoxvirus, Parapoxvirus, Suipoxvirus, and Yatapoxvirus. In contrast, the latter subfamily is known to infect invertebrates, and it is grouped into four genera: Alphaentomopoxvirus, Betaentomopoxvirus, Deltaentomopoxvirus, and Gammaentomopoxvirus[15]. At present, four kinds of orthopoxviruses are reported to be pathogenic to humans; the other three are cowpox virus (CPXV), vaccinia virus (VACV), and variola virus[16].

Structure

MPXV has similar morphological characteristics to other orthopoxviruses. By electron microscopy, the virus can be observed as an oval or brick-like particle with an outline size of about 200 nm × 250 nm. The core is a biconcave with a lateral body on each side[17]. There were structural proteins and DNA-dependent RNA polymerase in the virus particles, and the genome was double-stranded DNA, about 197 kb in length[18]. Unlike other viruses, MPXV does not replicate in the nucleus but in the cytoplasm, called the virus factory[19]. Four infectious viral particles can be distinguished during orthopoxvirus replication - intracellular mature enveloped virus particles, intracellular enveloped virus particles, cell-associated enveloped virus particles, and extracellular enveloped virus particles[20]. Both intracellular mature virus particles (mature virions, MVs) and extracellular enveloped virus particles (extracellular viruses, EVs) play an important part in pathogenesis[21].

MVs deliver EVs from viral factories to the site of wrapping in the juxta-nuclear region, where they are encapsulated by two additional lipid bilayers from the trans-Golgi network or endosomal cisternae to become intracellular enveloped virions (IEVs)[22]. IEVs are transported to the cell surface, where their outer membrane fuses with the plasma membrane to produce a double-wrapped cell-associated enveloped virion (CEV)[23]. These IEVs are MVs with an additional outer membrane. A few IEVs can be released from infected cells, but most IEVs are transported to the cell periphery of the cytoplasmic membrane to form double-encapsulated CEVs on the cell surface[24]. Following the membrane fusion event, IEV-associated viral proteins are embedded in the cell’s plasma membrane. A fraction of CEVs stimulates actin polymerization, forming actin tails[25-27]. Once CEVs are off the cell surface, they are called extracellular enveloped virions[28].

MPXV entry host cell process can be divided into six main steps: Binding, cell surface movement, signaling, internalization, intracellular transport, and membrane penetration[29]. The differences between MVs and EVs are mainly in three aspects: Binding, internalization, and membrane penetration[30]. During the binding step, the initial binding of MVs to cells is closely related to the glycosaminoglycans expressed on the cell surface. The virion proteins A27 L, D8 L, and H3 L on MV can bind to glycosaminoglycans to promote the attachment of MVs. Notably, genomic analyses of the 2022-2025 outbreak have revealed mutation signatures mediated by APOBEC3 cytidine deaminase activity, characterized by a significantly elevated C-to-T mutation rate[31]. This mutation pattern is relatively rare in orthopoxviruses and may be associated with the adaptive evolution of the virus in a new host population (humans). Although these mutations have not altered the clade classification of the virus (still belonging to the West African clade), some mutations are located in the epitope regions of viral immunogenic proteins (e.g., A29 L, B6R), potentially affecting the recognition efficiency of neutralizing antibodies and thus posing a potential challenge to the cross-protective efficacy induced by vaccination[24,32-34]. EVs have different viral epitopes from MVs. Therefore, they used different attachment factors, and it has been reported that phosphatidylserine (PS) binding to the serum protein Gas6 promotes EVs infection while having little effect on MVs infection. During internalization, MVS and EVs utilize macropinocytosis and require epidermal growth factor receptor signaling[35,36]. However, MV-induced macropinocytosis depends upon intact PS in the viral membrane, and EV does not depend on exposed PS to trigger micropinocytosis[37,38]. The final step of virus entry into cells is the fusion of the viral membrane and the cell membrane of endocytic organelles. EVs require low-pH acidification to destroy the surface membrane and promote membrane fusion, while low pH is unnecessary for MV-mediated infection[36,39].

Strain

Due to the greater stability of double-stranded DNA and the 3′-5′ proofreading exonuclease activity of poxvirus DNA polymerase, DNA genomes such as those of MPXV mutate significantly less frequently than RNA viruses[31]. MPXV is evolutionarily highly conserved from other RNA viruses and is divided into two clades, the West African and the Congo. The genomic sequence difference between the two strains is only 0.5%[40]. There is a difference in virulence between the Central and West African strains of MXPV. In an animal experiment, cynomolgus monkeys were challenged with high or low doses of monkeypox Central or West African strain[41]. Multiple clinical reports have also shown that West African monkeypox infections in human animals exhibit mild clinical symptoms. Their human case fatality rates are estimated at 10.6% or 3.6%[42]. To date, viruses isolated from cases in non-endemic countries have all belonged to the West African clade[43,44]. Although the virus was identified decades ago, human immunity to MPXV infection is poorly understood. Consequently, inferences regarding the interaction of MPXV with the host immune system are frequently derived from studies on VACV and related orthopoxviruses. The subsequent sections will examine the underlying mechanisms of MPXV-host immunity.

INNATE IMMUNE

The innate immune system is the first line of defense against pathogens. Together with the adaptive immune system, the innate immune system constitutes the host immune system. It plays a vital role in combating foreign pathogen infection[45]. The main early features of viral infection are the production of cytokines, type I interferon (IFN-I), and the activation of immune cells. Pattern recognition receptors initiate innate immune responses by recognizing the conserved molecular structure of pathogens, known as pathogen-associated molecular patterns[46].

Pattern recognition receptors

MPVX is a double-stranded DNA virus, and it can be recognized by sensors related to DNA recognition to initiate downstream signal transduction. So far, the identified DNA receptors mainly include Toll-like receptor (TLR), Retinoic acid-inducible gene I (RIG-I)-like receptor (RLR), and cytosol DNA sensors.

TLR: Currently, 10 TLRs have been identified in humans, conserved throughout the animal kingdom, and play an essential role in immunity in various species. Among them, TLR9 is the only known DNA sensor[47]. TLR9 recruits the adaptor protein MyD88, which then recruits the tumor necrosis factor (TNF) receptor-associated factor 6 and IκB kinase complex, activating nuclear transcription factor-kappaB (NF-kB) and producing inflammatory cytokines[48]. Even though TLR9 is the only TLR that recognizes DNA, other TLR family members play a role in detecting orthopoxvirus infection. VACVs are identified by TLR2 and enlist MyD88 to initiate the transcription of IFN-I and proinflammatory cytokines by utilizing TIRAP as an additional adaptor[49]. Furthermore, in animal studies, TLR2, TLR4, TLR6, and TLR7 have all been reported to be activated by MPXV or other poxviruses, leading to the production of IFN-Is, pro-inflammatory cytokines, and chemotactic factors[50].

RLR: The RLR-signaling pathway is one of the most important defense mechanisms against RNA viruses[51]. RLRs are comprised of RIG-I, melanoma differentiation-associated protein 5 (MDA5), and laboratory of genetics and physiology 2[52]. RIG-I reacts most strongly to negative-strand RNA viruses, such as influenza viruses, whereas positive-strand picornaviruses activate MDA5[53]. All three RLRS share a similar domain structure, including a central DExD/H box RNA helicase domain with the capacity to hydrolyze adenosine triphosphate and to bind and possibly unwind RNA, and a C-terminal regulatory domain[52]. MAD5 and RIG-I also have an N-terminal region of tandem caspase activation and recruitment domains (CARD)[54]. RIG-I and MDA5 endure conformational changes that expose and multimerize their CARDs, allowing homotypic CARD-CARD interactions with MAVS upon activation. MAVS is affixed to mitochondria, mitochondrial-associated membranes, and peroxisomes by its C-terminal transmembrane domain and relays the signal to TANK-binding kinase 1 (TBK1) and IκB kinase. Inducing the expression of IFN-Is and other genes. Due to the absence of an N-terminal CARD, the laboratory of genetics and physiology 2 primarily regulates the RIG-I and MDA5 signaling pathways[55]. According to a few reports, RLR signaling can recognize double-stranded DNA infection by recognizing RNA encoded by DNA viruses[54].

Cytosolic DNA sensors: Induction of type 1 interferon by sensing DNA in the cytoplasm is an important way of host defense against DNA viruses. Cytosolic DNA receptors currently studied include DAI [also known as Z-DNA-binding protein 1 (ZBP1) or Delta-like protein 1], DEAH box protein (DDX36/DHX9), DNA-dependent protein kinase (DNA-PK), interferon-gamma (IFN-γ)-inducible protein 16 (IFI16), cyclic GMP-AMP Synthase (cGAS), and absent in melanoma 2 (AIM2)[56]. After recognizing VACV signals, ZBP1 can associate with receptor-interacting serine/threonine-protein kinase 1 (RIPK1) and RIPK3 kinases via RIP homotypic interaction motif domain interactions. After recognizing DNA viral signals, ZBP1 can interact with RIPK1 and RIPK3 kinases. The RIPK3 complex activates caspase-8-dependent apoptosis, a parallel MLKL-mediated necroptosis and NLR family pyrin domain containing 3-mediated pyroptosis pathway. RIPK1 can activate the downstream NF-κB signaling pathway[57]. In addition, ZBP1 is a positive regulator of IFN-I by enhancing the binding of interferon regulatory factor 3 (IRF3) and TBK1[58].

AIM2 recognizes double-stranded DNA from pathogens and then assembles together with ZBP1, apoptosis-associated speck-like protein containing a CARD (ASC), caspase-1, caspase-8, RIPK3, RIPK1, and fas-associated death domain protein to form a large multiprotein complex that drives inflammatory cell death (PANoptosis)[59,60]. VACV-induced interleukin (IL)-1β release, caspase-1 cleavage, and IL-1β maturation were attenuated in macrophages lacking AIM2. Similar findings were observed when bone marrow-derived dendritic cells were tested[61]. IFI16 and AIM2 belong to the PYHIN protein family[62]. IFI16 recognizes a 70-bp sequence in VACV and interacts with STING to activate TBK1-dependent IFN-β production[63]. IFI16 can also recruit ASC and caspase-1 to activate the inflammasome, producing proinflammatory cytokines[64]. The cGAS - stimulator of interferon genes pathway is one of the most critical pathways in the host response to DNA viruses. After activation, cGAS catalyzes adenosine triphosphate and GTP to produce 2’3’-GMP-AMP, binding the second messenger circular GMP-AMP to STING. STING recruits TBK1 and IκB kinase β to phosphorylate IRF3[65]. IRF3 is then re-localized to the nucleus to induce IFN, thereby establishing an antiviral state, and STING also activates NF-κB. Ultimately, this leads to the production of proinflammatory cytokines and IFN-I[66]. The researchers found that cGAS-deficient mice do not produce type I IFN during VACV infection. Thus, cGAS is essential for immunity to Poxvirus[67]. The family that regulates DNA-mediated type I IFN production is composed of two subgroups: The DHX and the DDX[68]. DHX9 and DHX36 can interact with unmethylated CpG-DNA, increasing myd88-mediated IFN production and NF-kB-mediated activation of antiviral factors[69]. DNA-PK is a heterotrimeric protein consisting of Ku70, Ku80, and the catalytic subunit DNA-PKcs[70]. DNA-PK binds dsDNA breaks and plays a role in non-homologous end joining (NHEJ) and DNA sensing. DNA-PK can up-regulate IFN and cytokines upon binding to DNA through the Sting pathway[70].

IFN-I and proinflammatory cytokines

IFN-I: The downstream signal pathways of the above signal receptors were mainly concentrated in IRF3/IFR7, NF-kB, and ASC/NLR family pyrin domain containing 3 inflammasome. These signal pathways induce the production of downstream cytokines, mainly divided into IFN-I and proinflammatory cytokines. Interferons have several functions in activating the innate immune response to viral infection[71]. In animal studies, within 24 hours of infection with poxvirus, the transcript abundance of a large group of IFN-related genes increased by up to 27-fold. For example, these 20 genes include double-stranded DNA-activated protein kinase, signal transducer and activator of transcription (STAT1 and STAT2), myxovirus resistance (MX1 and MX2), and chemokines (CXC motif). Ligand 10 (IP-10) and 2’-5’ oligoadenylate synthetase (OAS 1, 2, and 3)[72]. In the clinic, IFN-β has been demonstrated to inhibit MPXV replication and spread[73]. In addition to IFN-I, Patricia observed that IFN-protected mice from MPXV infection. The production of IFN-g after infection was inversely correlated with inflammation during the disease[74]. The F3 L gene of MPXV encodes a protein homologous to the product of the E3 L gene in VACV. Genomic comparisons between MPXV and VACV reveal striking similarities between their genes. In response to IFN-Is, MPXV expresses a homologue of the VACV E3 protein, which suppresses interferon production by inhibiting phosphorylation through competitive binding to RLR sensors[75].

Proinflammatory cytokines: Cytokine storm is thought to correlate with the severity of monkeypox disease in humans. In MPX patients, by examining the serum samples of MPX patients, scientists found that IL-1, IL-2R, IL-4, IL-5, IL-6, IL-8, IL-13, IL-15, IL-17, and MCP-1 were increased[76]. Although proinflammatory cytokines cause body damage, some are also thought to limit virus replication. IL-1α effectively controlled viral replication in VACV-infected mice without forming pox lesions or activating memory responses[77]. MPXV encodes BR-209, an IL-1β binding protein that prevents IL-1β from binding to the IL-1 receptor[78]. TNF-α is a cellular inflammatory factor that initiates inflammation during infection. However, it has been found that the resistance of TNF-α to poxvirus infection is closely related to TNF-α production[79]. ECTV infection in TNF-α-deficient mice leads to excessive activation of STAT3, causing more substantial pathological damage[80]. However, MPXV and other orthopoxviruses inhibit the activation of NF-kB and TNF signaling pathways by encoding immunomodulatory proteins[81]. For example, Orthopoxviruses can produce a family of α-helical proteins, the B cell lymphoma-2 family, and these newly identified proteins play novel roles in antagonizing NF-κB and interferon signaling pathways and interfering with the release of proinflammatory cytokines[82].

Innate cellular immune

Innate immune cells play a crucial role in mediating innate immunity. Monocytes were the initial infection targets of MPXV, which can infiltrate inflamed or mucosal tissues and differentiate into macrophages or dendritic cells[83]. CD11b+ Ly6C + Ly6G + monocytes produce IFN-1 and ROS, which inhibit the spread of VACV in mice[84]. Dendritic cells are antigen-presenting cells. Poxvirus produces A39R protein, which binds to Plexin C1 to inhibit the phagocytosis of dendritic cells and neutrophils, which means to evade the immune system[85]. Monocyte-derived macrophages are considered a transport carrier for VACV[86]. However, in animal experiments, Murine alveolar macrophages have been shown to limit the replication of VACV[87]. The chemokine binding protein viral CC chemokine inhibitor secreted by MPXV inhibits the killing of MPXV by macrophages via inhibiting the chemotactic protein macrophage inflammatory protein-1 alpha[88]. In research on rhesus monkeys, a 23-fold increase in natural killer (NK) cells was found in the serum of rhesus monkeys infected with MPXV. The significance of NK cells in managing MPXV viral load was showcased in CAST/EiJ mice, which are at a higher risk of orthopoxvirus infection due to their low NK cell counts[89]. Furthermore, NK cells and Dendritic cells s can act as intermediaries between innate and adaptive immunity by activating CD8+ T cells during infection. MPXV produces specific proteins to bind directly to NK cell-activated receptors to trigger activation signals or to eliminate inhibitory signals by altering or reducing the interaction between NK cell-inhibitory receptors and their ligands, for instance, by downregulating Major Histocompatibility Complex class I (MHC-I) expression, in order to evade destruction by NK cells[90].

ADAPTIVE IMMUNE

The adaptive immune system acts through the expansion and functional maturation of T and B cell subsets. T cells are essential in the host's defense against viral infection. In animal experiments, CD4+ and CD8+ T cells have been shown to play an important role in defense against orthopoxvirus infection. MPXV produces a protein known as orthopoxvirus MHC class I-like protein (OMCP), which also downregulates MHC class I expression, thereby aiding the virus in evading recognition by T cells. This achieves immune escape by weakening the responses of both NK and T cells. MPXV engages in complex interactions with the host immune system to evade antiviral responses. It encodes proteins such as OPG027 and C7 L, which suppress T-cell activation and modulate immune pathways including TNF, IL-17, and NF-κB. These actions inhibit host antiviral activity and permit immune escape. Furthermore, the virus suppresses expression of antiviral genes such as IFIT1 and IFIT2, which inhibit viral mRNA translation, thereby counteracting host defences. MPXV also encodes inhibitors of complement enzymes, altering the adaptive immune response. Deletion of complement enzyme inhibitory sites may impact immune responses[91,92].

T cell

CD4+ T cells play an auxiliary and regulatory role in cell maturation and humoral immunity by recognizing pathogen-derived peptides presented by MHC-II. CD4-deficient mice could not make any switched Immunoglobulin G (IgG) response and a weaker (or absent) IgM response after infection[93]. In addition, CD4+ T cells also exhibit cytotoxicity in a perforin-dependent manner to eliminate monkeypoxvirus-infected cells[94]. Considering cross-immunity to orthopoxviruses, CD4+ T cells also play an important role in long-term immunity. The number of CD4+T cells was positively correlated with the titer of protective antibody after VACV infection[95]. Some CD4+ T cells can transform into memory CD4+ T cells after infection[96]. Experiments have shown that memory CD4+ cells can be maintained for up to 50 years after poxvirus vaccination, thus playing a protective role in humans[97]. Among the MPXV patients, human immunodeficiency virus patients showed more substantial susceptibility and severe disease course, especially CD4+ T cell counts < 200 cells/mm³[98]. These results suggest that CD4+ T cells play an important defensive role in MPXV infection. CD4+ T cells mainly differentiate into regulatory T (Treg) and T helper (Th) cells. Helper cells mainly contain Th1, Th2, and Th17[99]. Clinical reports show that the Th1 cell response is weakened, and the Th2 helper cell response is enhanced after MPXV infection[76]. The results in vitro indicate that ECTV reduced the ability of dendritic cells to activate/polarize Th1 and Th2, thereby reducing the adaptive antiviral immune response[100]. The balance of Treg and Th17 is important in controlling the inflammatory response. Current studies have found that Treg deficiency can lead to the reduction of IFN-γ and IL-2 secretion and the increase of IL-4 secretion during VACV infection, thereby affecting the differentiation of Th cells and damaging the adaptive immune function[101]. Effector CD8+ T cells clear the virus by accumulating large numbers in the infected lung and secreting molecules that kill virus-infected cells, such as IFN-γ[102]. The research shows tumor necrosis factor-related apoptosis-inducing ligand and IFN-γ-mediated pathways contributed to reducing virus-induced morbidity and mortality in a model using VACV infection of the mouse respiratory tract, in which IFN-γ produced by CD8+ T cells was crucial[103]. CPXV can inhibit the antiviral response associated with CD8+ T cells by inhibiting MHC class I molecule transport[104]. In contrast, MPXV can trigger the non-response state of T cells to achieve immune evasion by contacting infected cells with T cells[105]. γδT cells are unconventional lymphocytes with many characteristics of innate immune cells. It is possible to respond to pathogen-free molecular patterns and cytokines without TCR ligands[106]. In MPXV infection, γδT amplifies rapidly and can limit pathological damage to the tissue of infection[107].

B cell

The immune response involving B cells and immunoglobulins is vital for immunity against poxviruses. The specific B-cell response generated by VACV stimulation can control infection by MPXV because MPXV shares homology with other Poxviruses[95]. Currently, 14 MPXV proteins are bound by neutralizing antibodies generated by VACV vaccines[108]. B cells can produce immunoglobulin. After vaccination, virus-specific memory B cells initially declined but reached a plateau of about 1/10 times the peak and remained stable for 50 years at a frequency of about 0.1% of circulating IgG (+) B cells[109]. In B cell-deficient mice, vaccination did not protect against poxvirus infection[110].

These persistent memory B cells are functional and can mount a robust memory antibody response upon revaccination[111]. In a study of 200 MPXV-infected patients recruited in the Democratic Republic of the Congo between March 2007 and August 2011, those with IgM and IgG responses were 5.09-fold more likely to have severe lesions than those with only IgG responses[112]. Similarly, among the patients in the 2003 MPXV outbreak in the United States, the patients with moderate/severe disease had higher overall titers of anti-orthopoxvirus IgM compared with those with mild disease, and those with moderate/severe disease had much reduced and less frequent anti-orthopoxvirus IgG responses[112-114]. The inability of Dryvax immunization to protect against the production of MPXV in immunodeficient macaques is believed to be linked to the absence of a fully developed B cell response, as well as defective IgM to IgG isotype switching under conditions with CD4+ T cell helpers[95]. According to scientists, an IgG-only response indicates high levels of cross-protective IgG+ memory B cells, resulting in a primary and secondary response to a pathogen. In contrast, lacking such memory necessitates an IgM-dominated primary response, which is less effective in preventing disease. Therefore, an IgM response may be a disease severity biomarker[115-117].

Beyond virological characteristics, effective epidemic control also relies on robust diagnostic capabilities. Reverse transcription quantitative real-time PCR serves as the gold standard for MPXV diagnosis, yet its dependence on specialised infrastructure limits its practicality in resource-constrained settings. Vaccination remains the most effective control measure against MPXV to date. Over 40 monkeypox vaccine candidates are currently under development, encompassing multiple technological platforms including mRNA, subunit, and DNA vaccines. Regarding MPXV treatment, Brincidofovir, Tecovirimat, and Cidofovir have all demonstrated promising potential in clinical trials[118-122].

CONCLUSION

The current outbreak of MPXV is not a major cause for concern, as smallpox vaccination provides homologous immunity. Therefore, smallpox vaccination should be resumed to combat the threat of pandemics. Considering the present environment where pandemics are a significant threat, resuming smallpox vaccination is advisable. Although monkeypox belongs to the dsDNA virus and mutates with a probability less than that of the RNA virus, MPKV encodes various regulatory proteins to evade the monitoring and killing of the immune system, so we should be vigilant. More studies on innate and adaptive immune responses to MPXV are needed (Table 1)[49,52,54-56,63,67,69,76,123-138]. Currently, poxvirus-based vector systems developed based on VACV, cancer vaccines, and oncolytic immunotherapy have shown good development prospects[115,116]. Additionally, more research into the innate and adaptive immune responses and immune evasion mechanisms of MPXV and other related viruses may assist in developing effective vaccines and therapeutics. While smallpox vaccination provides cross-protective immunity, waning population-level immunity and the emergence of APOBEC-driven mutations necessitate renewed vaccine strategies. Continued research into host-MPXV interactions will inform next-generation vaccines and immunotherapeutics.

Table 1 Pattern recognition receptors sense poxviruses in different cells.

	PRR	Cell type/species	Virus	Ref.
TLR	TLR2	Dendritic cells, T cells, monocytes	VACV	Zhu et al[49], 2007; O'Gorman et al[123], 2010; Samuelsson et al[124], 2008
	TLR3	Macrophage	VACV, ECTV	Hutchens et al[52], 2008; Struzik et al[125], 2010
	TLR4	Macrophage	VACV	Hutchens et al[54], 2010
	TLR7	Dendritic cells	FWPV, VACV	Lousberg et al[55], 2010; Cao et al[126], 2010
	TLR8	Dendritic cells	VACV	Martinez et al[56], 2010
	TLR9	Dendritic cells	VACV, ECTV	Wang et al[127], 2010
RLR	RIG-I	PBMC, macrophage, Hela	SPPV, MYXV, VACV	Pichlmair et al[128], 2009; Pollpeter et al[129], 2011; Georgana et al[130], 2018
	MAD5	Hela	VACV	Oliveira et al[131], 2020
	LGP2	Embryonic fibroblasts	VACV	Cheng et al[132], 2018
Cytosol DNA sensors	ZBP1	Fibroblasts	VACV	Balachandran et al[63], 2021
	AIM2	Dendritic cells, macrophages	VACV	Rathinam et al[67], 2010
	IFI16	Macrophage	VACV	Balachandran et al[69], 2018
	Sting	Macrophage	VACV	Georgana et al[133], 2018
	cGAS	Macrophage, dendritic cells	FWPV, VACV, ECTV	Oliveira et al[134], 2020; Cheng et al[135], 2018; Schoggins et al[136], 2014; Dai et al[137], 2014
	DHX9	A549	MYXV	Rahman et al[138], 2021
	DNA-PK	Embryonic fibroblasts	VACV	Balachandran et al[63], 2018

PRR: Pattern recognition receptor; TLR: Toll-like receptor; RLR: RIG-I-like receptor; VACV: Vaccinia virus; ECTV: Ectromelia virus; FWPV: Fowlpox virus; PBMC: Peripheral blood mononuclear Cell; RIG-I: Retinoic acid-inducible gene-I; MAD5: Melanoma differentiation-associated protein 5; LGP2: Laboratory of genetics and physiology 2; ZBP1: Z-DNA binding protein 1; AIM2: Absent in melanoma 2; IFI16: Interferon gamma inducible protein 16; cGAS: Cyclic GMP-AMP synthase; DHX9: DExH-box helicase 9; SPPV: Sheeppox virus; CPXV: Cowpox virus; MYXV: Myxoma virus; DNA-PK: DNA-dependent protein kinase.