Introduction

Murine gammaherpesvirus 68 (MHV68, also referred to as γHV68 and MuHV4), a large double-stranded DNA virus of 118 kb length, is a natural pathogen of murid rodents [1, 2]. MHV68 shares significant genetic similarities with the human gammaherpesviruses Epstein-Barr virus (EBV) and Kaposi’s sarcoma–associated herpesvirus (KSHV) and furthermore displays comparable pathogenic strategies [3]. Due to the narrow host tropism of EBV and KSHV, and the problems encountered upon using chimeric mice harboring human tissues as model systems, the key pathogenic determinants of gammaherpesvirus infections have remained largely obscure. Fortunately, due to the significant genetic and biological similarities between MHV68, EBV, and KSHV, which include, but are not restricted to, the establishment of latency in B cells and propensity to drive tumorigenesis, coupled with the ability of MHV68 to readily infect laboratory mice (Mus musculus) altogether make MHV68 a strong system for understanding the key pathogenic determinants of in vivo gammaherpesvirus infections [3].

MHV68 infection has two phases, acute and chronic [3, 4]. The acute phase last 10–12 days, while the virus undergoes lytic replication beginning in mucosal epithelial cells and macrophages. The virus with the help of dendritic cells, eventually makes its way to the draining lymph nodes and from there is disseminated to other sites such as the spleen and peritoneal cavity [3,4,5]. Once the virus has reached the secondary lymphoid tissues, it begins its switch to viral latency by infecting naïve B cells in order to establish its latent viral reservoir in memory B cells [6,7,8,9,10]. The virus achieves this by infecting naïve B cells and inducing a robust germinal center response which includes both virus-infected and uninfected B cells [6, 7] that subsequently differentiate into class-switched plasma cells, where viral reactivation occurs, or memory B cells that host life-long latent infection [8]. The peak of viral latency, where the germinal center response and levels of latent virus are at their highest, is usually observed between 15–18 days post infection and throughout the remainder of the host life the virus can reactivate from latency to lytic replication to maintain the latent viral reservoir [3, 4]. MHV68 infection represents an ideal platform to study the impact of the immune response on gammaherpesvirus latency. Here, we highlight immune factors that regulate MHV68 latency. (Fig. 1) (Table 1).

Fig. 1
figure 1

Schematic of the host immune response during MHV68 latency. Summarized here are the various host-immune effectors that participate in regulating MHV68 latency. (Created with Biorender.com)

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Table 1 Host immune factors regulating MHV86 Latency
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TLR Signaling

Toll-like receptors (TLRs), representing a class of pattern recognition receptors (PRRs), are the first line of sensors against microbial agents including viruses and bacteria that are located on the host cell surface and intracellularly in endosomes. Although 12 murine TLRs, ranging from TLR1-9 and TLR 11–13, have been described till date, only a few have been characterized during MHV68 infection. TLR7 and TLR9 are two important TLRs which play a significant role in detecting and controlling MHV68 latency. Accumulated evidences suggest that latently infected TLR9−/− and TLR7 and TLR9 double-knockout splenocytes exhibit enhanced reactivation and latent viral load [11•]. Moreover, IFN-α response to MHV68 infection is shown to be completely abrogated in TLR7 and TLR9 double-knockout plasmacytoid dendritic cells (pDCs) [11•]. Taken together, the data suggests that TLR7 and TLR9 exhibit cooperativity while detecting and controlling MHV68 latency. The majority of TLRs propagate signals through a key adaptor protein MyD88 [12]. MyD88, once activated, recruits various kinases to induce a wide-spectrum signaling cascade that leads to the phosphorylation and activation of multiple transcription factors including nuclear factor-kB (NF-kB), IRF-3, and IRF-7, all factors which help control viral infection [13, 14]. Surprisingly however, there is a discordant view that proposes a proviral role of TLR signaling through the involvement of MyD88. This becomes evident from the report that highlights the involvement of MyD88 in establishing MHV68 latency in B cells, as demonstrated by a tenfold decrease in latent viral load in MyD88−/− mice [15]. More extensive studies are therefore warranted to determine the basis for the pro and anti-viral role of TLR and subsequent MyD88 signaling during MHV68 infections.

NF-κB Activity

NF-κB is an important signaling pathway comprised of multiple components, including p50, p65 (RelA), and IkBα, that plays a significant role in inflammation [16]. Importantly, NF-κB signaling plays a key role in establishing MHV68 latency in B cells [15] while its inhibition is reported to induce MHV68 reactivation from latency [17]. The importance of NF-κB signaling activation in establishing latent MHV68 infection is further substantiated by the study that highlights the inability of MHV68 mutant IkBα (IkBαM), which expresses IkBα and prevents NF-κB signaling activation, to establish MHV68 latency in the spleen [18]. A more definitive role of NF-kB signaling in suppressing MHV68 reactivation comes through studies carried out on another NF-kB subunit p50. The study involved the use of p50−/− mice. Additionally, mixed bone marrow chimeric mice reconstituted with both p50-sufficient and p50-deficient bone marrow were used in the study [19]. Unlike wild-type infected mice, infections in p50−/− and mixed bone marrow chimeric mice resulted in persistent virus replication in the lungs, suggesting a cell-intrinsic role of p50 in suppressing MHV68 reactivation from latency.

Although the requirement of NF-κB signaling activation for promoting MHV68 latency is widely accepted, some discordant observations do exist. It is reported that the latency-maintenance gene ORF73/mLANA triggers NF-kB subunit p65 degradation, through the involvement of SOCS-box motif in mLANA [20]. This motif can target ubiquitination and proteasomal degradation of p65 through assembly of an ElonginC/Cullin5/SOCS-like E3 ligase complex. Surprisingly, this function is shown to be required for the efficient latency establishment [20]. As such, mutations in the MHV68 mLANA protein that disrupt this function prevent efficient establishment of MHV68 latency. The underlying mechanisms for these contrasting observations have not been thoroughly investigated. Furthermore, non-canonical NF-kB signaling is also reported to contribute significantly to the establishment of MHV68 latency. Mice lacking the B cell-activating factor receptor (BAFF-R), one of the activators of non-canonical NF-κB signaling, exhibit defects in establishment of MHV68 latency in secondary organs [21]. A recent finding by Cieniewicz et al. also supports the role of non-canonical NF-kB signaling in establishment of MHV68 latency. The study utilizes recombinant virus expressing a dominant negative form of IkB kinase α (IKKα), named IKKα-SA to prevent phosphorylation by NF-κB -inducing kinase (NIK) and render NF-κB inactive. IKKα-SA expression significantly reduced the viral latency in comparison to wild-type at 16 days post infection (dpi) [22••].

Interferons and Interferon Regulatory Factors (IRFs)

Interferons (IFNs) constitute a critical host antiviral defense system comprising of type I, II, and III IFNs [23,24,25]. While Type I IFN family, consisting of various cytokines, signal through the ubiquitously expressed interferon a/b receptor (IFNAR), Type II IFN, represented by the single cytokine IFNγ, signals via the interferon gamma receptor (IFNGR). Both Type I and Type II IFNs are ubiquitously expressed. However, expression of type III IFN receptors is restricted to mucosal epithelial tissues and hepatocytes [26] and its role in gammaherpesvirus infection is largely unknown.

Type I IFN signaling represses MHV68 lytic replication, inhibits persistent replication, and attenuates reactivation from latency during chronic infection. These observations reveal that the antiviral role of type I IFN is maintained throughout all stages of gammaherpesvirus infection [27,28,29]. Similarly, lack of type II IFN increases MHV68 persistent replication and viral reactivation during chronic infection in peritoneal macrophages, but not B cells [30, 31].

Downstream signaling of all three IFN types requires STAT1-containing transcriptional complexes for inducing expression of largely overlapping interferon stimulated genes (ISGs) [23, 32, 33]. Studies carried out in MHV68 infected mice reveal that type I and II IFN signaling and STAT1 expression suppress the establishment of latency in vivo [30, 31, 34,35,36,37]. Further, during the establishment of MHV68 latency, myeloid cell-specific STAT1 expression helps restrict persistent replication in the lungs and viral reactivation in the peritoneal cavity [38]. Contrary to the expected antiviral role of STAT1, a recent study reveals that T-cell specific STAT1 activity promotes acute and persistent gammaherpesvirus replication in the lungs and the establishment of latent viral reservoir in B cells by rendering the B cells more permissive to MHV68 infection [39•]. The recent studies on the cell type specific role of STAT1 in the control of MHV68 latency reveal a diverse and multifaceted role for STAT1 during MHV68 infection, highlighting the need for further study into the role of STAT1 in MHV68 latency.

Interferon regulatory factors (IRFs), representing a family of transcription factors, typically drive an antiviral response in the host cells. IRF-1 mostly plays an anti-viral role during MHV68 infection. Global IRF-1 expression opposes the MHV68 latent reservoir and restricts viral reactivation in both the spleen and peritoneal cavity. Further, it restricts the MHV68-driven germinal center response in splenic B cells [40]. Interestingly, recent studies into the cell intrinsic effects of IRF-1 found conflicting anti-viral and proviral attributes of IRF-1. B cell-intrinsic IRF-1 was found to support the establishment of chronic infection as loss of IRF-1 specifically in B cells led to significant reductions in viral latency and reactivation in the spleen as well as attenuation of the MHV68-driven germinal center response [41•]. B cell-intrinsic IRF-1 was found to promote the expression of the tyrosine phosphatase SHP1 in B cells, which is needed to support the MHV68-driven germinal center response and subsequent establishment of peak latency [41, 42]. The role of T cell-intrinsic IRF-1 was found to be more in line with what was observed in the global IRF-1−/− as loss of IRF-1 in T cells led to increase in the MHV68-driven germinal center response as well as viral latency and reactivation in the spleen and peritoneal cavity [43••]. One potential mechanism to explain the anti-viral role of T cell-intrinsic IRF-1 was its function in CD8 + T cells since naive global IRF-1 deficient mice exhibit decreased levels of CD8 + T cells [44]. Surprisingly, no such decrease in the number or the function of MHV68-specific CD8 + T cells was observed in infected global IRF-1−/− or T cell specific IRF-1−/− mice. The evidence indicates that the loss of IRF-1 in T cells during chronic MHV68 infection significantly increases interleukin-17A (IL-17A)-expressing CD4 + T cells [43••]. IL-17A signaling was recently found to promote the MHV68-driven germinal center response and the establishment of viral latency [45••]. The T cell-intrinsic IRF-1 study identified a novel link between IRF-1 and IL-17 responses [43••]. Further, in light of the proviral role of IL-17A signaling during chronic MHV68 infection, the study altogether provides a novel mechanism for T cell-intrinsic IRF-1 mediated suppression of CD4 + T cells to restrict chronic MHV68 infection [43••].

IRF-3 is a ubiquitously expressed transcription factor having multiple direct target genes. IRF-3 exhibits an antiviral function by attenuating acute replication of a various viruses, including MHV68. However, a recent study demonstrates an unexpected proviral role of IRF-3 during the latency phase, wherein IRF-3 aids in the establishment of MHV68 latency in splenic B cells, particularly in germinal center and activated B cells [46•]. The study demonstrated that IRF-3−/− mice, in comparison to control C57BL/6 J (BL6) mice, had a 25-fold reduction in viral latency, whereas the frequencies of MHV68 reactivation were similar in BL6 and IRF-3−/− splenocytes, indicating a greater efficiency of viral reactivation in the absence of IRF-3. Further, loss of IRF-3 led to a tenfold decrease in the efficiency of MHV68 to infect germinal center B cell, indicating that IRF-3 supports the efficient infection of B cells, which directly supports the establishment of latency [46•].

Unlike IRF-3, IRF-7 attenuates chronic infection by restricting the establishment of MHV68 latency in the peritoneal cavity and, to a lesser extent, viral reactivation in the spleen [47•]. It was observed that the infection of IRF-7−/− mice resulted in a 9.8-fold increase in viral latency in the peritoneal cavity and viral reactivation [47•]. Further, loss of IRF-7 during MHV68 infection led to significantly higher levels of latently in infected B-cells and to a much lesser extent non-B cells in the in peritoneal cavity [47•]. Indicating that IRF-7 is antiviral in peritoneal B-cell and non-B-cell populations. However, viral latency in the spleen of IRF-7−/− mice was similar to BL6 mice [47•]. These findings suggest that IRF-7 has differential effects in peritoneal cells vs splenic cells, particularly B cells. IRF-7 does not affect splenic latency, suggesting that IRF-7 has a minimal role in germinal center B cell and other splenic B-2 B cell subsets. A subset of B cells called B-1 B cells are the major reservoir of latent virus in the peritoneal cavity [48] and this study would suggest that IRF-7 plays a more significant antiviral role in B-1 B cells than B-2 B cells as it restricts the establishment of latency as well as viral reactivation in the peritoneal cavity [47•].

Epigenetic Control

DNA methyltransferases represent a critical component of the innate host response to clear lytic infections. Their concomitant role in regulating MHV68 latency is also reported. A study by Gray et al. revealed that DNA methyltransferases, Dnmt3a and Dnmt3b, promote viral latency [49]. They found that the mice lacking Dnmt3a and Dnmt3b in B lymphocytes displayed prolonged acute replication that persisted up to 42 days post-infection, thereby delaying the onset of latency. Importantly, these effects correlate with hypomethylation of an essential lytic switch gene 50 promoter which controls the interchange between lytic and latent gene expression programs [50]. In addition to DNA methyltransferase, histone modifications also influence innate immune response toward MHV68 latency. Pertinently, data generated from infected macrophages reveal that the MHV68 lytic cycle is silenced and latency establishment prevented due to the inhibitory action of histone deacetylases (HDACs)- HDAC3 and HDAC4, and the nuclear receptor corepressor (NCoR) acting on the essential MHV68 lytic switch, gene 50, expression [51].

Cell Death and Autophagy

Apoptosis (programmed cell death) constitutes a critical component of the cell-intrinsic (innate) as well as an adaptive immune response during virus infection [52]. Caspase 3, a central proteolytic mediator of apoptosis, has been reported to help regulate MHV68 latency. Studies demonstrate that the mice lacking caspase 3 exhibit increased reactivation of MHV68 and increased latent viral load [53]. Further, as an immune evasive strategy, MHV68 gene, M8, encodes a mitochondria-targeted inhibitor of apoptosis (vMAP). vMAP interacts with cellular anti-apoptotic proteins Bcl-2, Bcl-XL, and the mitochondrial voltage-dependent anion channel (VDAC-1) to inhibit apoptotic pathways [54].

In addition to M8, another MHV68 gene, M11, encodes Bcl-2 ortholog (vBcl-2) that is expressed during latency and inhibits BH3-containing proapoptotic cellular Bcl-2 family members [55, 56]. In vivo studies demonstrate that MHV68 mutants with defective vBcl-2 gene are incapable of establishing efficient latency [57,58,59]. The studies demonstrate that vBcl-2 blocks B cell receptor-mediated apoptosis to subvert the negative selection and promote the survival of the developing B cells, which are the major sites for viral latency [60].

In addition to apoptosis, the DNA Damage Response (DDR) also acts as a cell-intrinsic sensor of herpesvirus infection. DDR triggers the phosphorylation of ataxia-telangiectasia mutated (ATM) which activates it, leading to subsequent phosphorylation of H2AX [61]. Activated ATM is involved in regulating chronic gammaherpesvirus infections. Interestingly, ATM is reported to mediate both pro and anti-viral activities in an immunocompromised host. ATM deficient mice infected with MHV68 show increased reactivation and persistent replication [62]. Contrarily, in vitro data generated from primary macrophages reveal that ATM promotes MHV68 replication [63]. To explain this conundrum, studies were conducted in a mouse model having ATM deficient myeloid cells. While ATM deficiency, in the myeloid cells, did not affect gammaherpesvirus-specific T cell response, it suppressed MHV68 reactivation and latency [64]. Similarly, a study in ATM deficient B cells found that ATM also acts in a proviral manner within B cells to support viral reactivation and indirectly facilitate establishment of viral latency [65]. These studies suggest that ATM regulates chronic gammaherpesvirus infection by balancing its proviral functions within myeloid cells and B cells and its antiviral functions within the virus-specific T cell immune response [62, 64, 65].

Autophagy is a multistep degradation process in eukaryotes that involves catabolic lysis of the cytoplasmic contents for nutrient replenishment and subsequent maintenance of cellular homeostasis [66]. In addition, autophagy also mediates intracellular sequestration and clearing of bacterial and viral infections [67]. Notably, vBcl-2, like host (cellular) Bcl-2 (cBcl-2), binds the pro-autophagy molecule Beclin-1. However, this binding has a higher affinity than that between cBcl-2 and Beclin-1 [68]. The vBcl-2-Beclin-1 interacting complex potentiates autophagy inhibition during MHV68 infection [69]. Studies confirm that the MHV68 containing the vBcl-2 mutant MHV68-vBcl-2Δα-1 exhibits significantly decreased latent viral load at later times of latency (28–42 days post-infection). Altogether, the available data highlights the antagonist role of vBcl-2 for both autophagy (required for efficient maintenance of viral latency infected cells) and apoptosis (required for efficient reactivation from latency). Besides Beclin-1, other autophagy genes including Fip200, Atg14, Atg16l1, Atg7, Atg3, and Atg5, are also reported to facilitate MHV68 reactivation from latency in macrophages [70].

Interleukins

Various cytokines including interleukin-1 (IL-1), IL-4, IL-6, IL-10, IL-16, IL-17A, and IL-21 play some role in MHV68 latency establishment. A study carried out by Darrah et al. found a proviral role of IL-1 signaling during chronic MHV68 infection. They confirm the participation of IL-1 in the reactivation from latency in the spleen, promoting the virus-driven germinal center response, and facilitating the virus-specific responses along with irrelevant class-switched humoral responses [71•]. Conversely, however, Sylvester et al. observed that IL-1 signaling switches the role from proviral to antiviral during the long-term stage of the chronic MHV68 infection [72•]. Moreover, while global IL-1 signaling promoted antiviral CD8+ T cell response to suppress MHV68 reactivation from peritoneal cells of long-term infected host, MHV68 infection in a T cell-specific IL-1 Receptor 1 (IL-1R1) deficient mouse model failed to reproduce the results, suggesting that T cell intrinsic IL-1 signaling was not required for antiviral T cell responses or control of viral reactivation [72•]. Together, the study reveals a dynamic nature of host IL-1 signaling in regulating chronic gammaherpesvirus infection.

To further highlight the role of cytokines in regulation of MHV68 latency, IL-4 has been found to induce MHV68 reactivation by inducing STAT6 to bind the gene 50 promoter in latently infected macrophages but not in B cells, suggesting that IL-4 regulated MHV68 latency and reactivation is cell type specific [73]. Further, although, IL-6 is produced at high levels during MHV68 infection, it is not essential for the development of an effective immune response to MHV68. Studies carried out in IL-6−/− mice demonstrate that replicating virus could be eliminated from the respiratory tract within 15 days of infection. Similarly, no significant change in the levels of latent virus in IL-6−/− mice compared to control mice were observed, indicating that IL-6 does not influence viral replication and latency [74]. In contrast, IL-10 is proposed to play a dichotomous role during MHV68 infection. While IL-10−/− mice infected with MHV68 exhibit decreased viral load in the spleen, the absence of IL-10 promotes IL-12 production as well as increased splenomegaly. Altogether, the data suggest the involvement of IL-10 in protection as well as pathogenesis of disease [75]. Moreover, involvement of latency-specific MHV68 M2 protein in enhancing serum IL-10 levels in B cells at 2 weeks post-infection further implicates IL-10 in establishment of MHV68 latency [76].

Another important cytokine implicated in MHV68 latency is IL-16. Cytokine array analysis reveals enhanced expression of IL-16 in MHV68-immortalized B cells. Furthermore, the data obtained from both in vitro as well as in vivo studies suggest the involvement of IL-16 in maintenance of MHV68 latency with no significant role in lytic replication. IL-16 contributes to the maintenance of MHV68 latency by inhibiting viral reactivation. The inhibition of viral reactivation and the subsequent maintenance of latency is achieved by the inhibitory action of IL-16 action on Tyr705 STAT3- dephosphorylation and p21 expression [77•].

Importantly, a recent study demonstrates the involvement of host IL-17A, a member of the IL-17 cytokine family, in the establishment of chronic gammaherpesvirus infection and gammaherpesvirus-driven germinal center response, while having no impact on the acute phase of infection [45••]. The study reports a significant reduction of chronic MHV68 infection and virus-driven germinal center response in the absence of IL-17 receptor A (IL-17RA). Accordingly, these findings propose a proviral role of host IL-17A signaling during gammaherpesvirus infection [45••]. Consonantly, a more recent study demonstrates that T cell-specific IL-17RA signaling supports the MHV68-driven germinal center response by supporting CD4 + T follicular helper cell expansion, the establishment of latency in the spleen, and viral reactivation in the spleen and peritoneal cavity [78•].

Finally, the cytokine IL-21, produced by T follicular helper (Tfh) cells, has been shown to play an important role in both the maintenance of the germinal center response as well as establishment of MHV68 latency. IL-21 receptor (IL-21R) deficient mice infected with MHV68 shows impaired latency [79]. Furthermore, loss of IL-21R during MHV68 infection leads to reduced expansion of infected B cells. Also, the presence of fewer infected plasma cells in IL-21R−/− mice causes a significant reduction in the frequency of splenocytes capable of reactivating the virus. Taken together, these data confirm multiple roles of IL-21 signaling during MHV68 infection and identify IL-21 as a critical Tfh cell-derived cytokine for efficient establishment of gammaherpesvirus B cell latency [79].

Viral Kinase: Orf36

MHV68 protein orf36 is a conserved protein kinase that has homologs in both EBV (BGLF4) and KSHV (orf36). These kinases have been shown to facilitate reactivation and lytic replication in all three gammaherpesviruses [71, 80,81,82,83]. Regarding latency, orf36, expression and enzymatic activity were found to be critical for the establishment of viral latency, viral reactivation, the robust virus-driven germinal center response, and the irrelevant B cell response during the establishment of chronic infection [71•]. Further, studies suggest that expression and enzymatic activity of orf36 facilitate the splenic latency through the induction of DNA Damage Response [58]. Conversely, research carried out by the Tarakanova lab has uncovered that orf36 has function independent of enzymatic activity as well as an H2AX (a core histone 2A (H2A) variant) associated role of orf36 in the early MHV68 latency establishment [84]. Orf36 interacts with IRF-3 and suppresses its ability to activate IFN-αβ transcription [83]. However, that interaction does not seem to be necessary for the ability of orf36 to promote the establishment of latency as loss of IRF-3 in mice infected with a kinase null-MHV68 mutant neither rescues viral latency nor the virus-driven germinal center response [46•]. Another recent study unveiled an antagonistic relationship between MHV68 orf36 and IRF-1 [85•]. The study demonstrated that global IRF-1 deficiency significantly rescues the attenuated latent reservoir of the orf36-null MHV68 mutant during chronic infection while having a minimal effect on the attenuated lytic replication [85•]. In another study, myeloid-specific STAT1 deficiency is shown to rescue the attenuated splenic latent reservoir of the kinase-null MHV68 orf36 mutant [38]. Altogether, these data highlight the multifunctional nature of MHV68 orf36 during latency.

Conclusion

MHV68 presents a highly tractable system for examining gammaherpesvirus latency in an intact host. This review discusses recent discoveries made within the MHV68 system that highlight the way in which the immune system can impact viral latency and reactivation (Fig. 1) (Table 1). MHV68 has co-evolved with its host and has developed mechanisms to counteract the host immune response. It employs various immune evasion strategies to avoid elimination and subsequently establish a lifelong latent infection. MHV68 orf36, for instance, interacts with STAT1 in myeloid cells to antagonize its antiviral functions in order to access splenic B cells and optimally establish latency [38]. Further, MHV68 has usurped multiple other immune functions that are classically anti-pathogen and uses them in a proviral way to facilitate the establishment of viral latency. For instance, adaptor protein MyD88 plays a vital role in mediating a proviral role of TLRs during the establishment of MHV68 latency in B cells [15]. Similarly, in contrast to the widely accepted antiviral role of type I and II IFN signaling and STAT1, T cell-specific STAT1 activity is shown to promote acute and persistent gammaherpesvirus replication in the lungs and helps establish a latent viral reservoir in B cells [39•]. Finally, MHV68 utilizes IL-17RA signaling, which is critical for the clearance of multiple bacterial and fungal infection, to promote the efficient establishment of viral latency [45••]. Therefore, these counterintuitive, proviral functions of MyD88, STAT1, IL-17RA, and other immune functions highlighted in this review warrants more thorough and in-depth analysis in future to determine the underlying mechanisms governing these diverse factors in promoting MHV68 latency.

Despite the multiple immune factors which promote MHV68 latency, there are still many immune factors that restrict and control MHV68 latency. Type I and II Interferons [27,28,29,30,31] and IRFs such as IRF-1 [40, 43, 47, 69], it is not yet clear how this might be achieved. Further, the discordance between latency-maintenance gene ORF73/mLANA and NF-κB p65 subunit during viral latency establishment [20], as well as contradictory role of MyD88 signaling in regulating viral latency and reactivation [15], requires more precise characterization of the differentiation of newly infected naive B cells into germinal center, memory, and plasma B cells.

These recent discoveries highlighted here uncovered some surprising and seemingly contradictory mechanisms that highlight the complexity of MHV68 latency. Moving forward it is clear that there is still much to learn in regard to how the immune response interacts with MHV68 to control viral latency and how MHV68 manipulates the host immune response for its own gain. Better understanding of the establishment of MHV68 latency, control of viral reactivation, and the host mechanisms that impact both will hopefully be instrumental to the development of targeted therapeutics against the human gammaherpesviruses, EBV and KSHV, to prevent gammaherpesvirus associated disease.