according to the paradigm of antigen processing by the antigen presenting cells ( apc ) , peptides arising from intracellular proteins are presented via class i mhc molecule .
this occurs after they are degraded via proteasome and transported by transporters associated with antigen processing ( taps ) .
differently , peptides that originate from extracellular antigens are delivered to the late endosomal / lysosomal compartment , where they are degraded by the lysosomal proteases and then presented in association with class ii mhc . in the lysosomes ,
the invariant chain , which blocks class ii mhc , is also degraded , rendering the mhc molecules available for peptide loading .
besides the differences in the sites of origin and processing , peptides presented via class i or ii mhc activate different populations of t lymphocytes , which are cd8 or cd4 , respectively .
however , exceptions to this model have been described and , for example , dendritic cells ( dcs ) , the most powerful apc , are able to present extracellular antigens also via class i mhc .
this event , known as cross - presentation , allows dcs to activate both cd4 and cd8 t cells , in response to an extracellular antigen . on the other hand ,
antigens of intracellular origin can be presented also in class ii mhc , after being delivered to the lysosomes , through double membrane vesicles , called autophagosomes .
autophagosomes are formed during the induction of autophagy , a self - eating mechanism through which cells recycle their own constituents and survive in stressful conditions .
some of them , such as atg1 , atg11 , and atg13 , regulate autophagosome formation ; others ( e.g. , atg2 , atg9 , and atg18 ) are required for membrane flow to the expanding phagophore .
the vesicle nucleation is instead dependent on the class - iii phosphatidylinositol 3 kinase ( ptdins3k ) complex formed by vps34 , vps15 , vps30/atg6 , and atg14 .
atg6 , also named beclin 1 , represents a protein with a pivotal role in the autophagy induction   and the microtubule - associated protein light chain 3 ( lc3 ) , or atg8 , is a marker of the autophagic vacuoles .
lc3 is expressed as full - length cytosolic protein and upon autophagy induction is cleaved by atg4 to form lc3i , which is then conjugated to phosphatidylethanolamine ( pe ) , generating lc3ii .
this molecule , whose formation indicates autophagy induction , is associated with the internal and external membrane of autophagosome and , by interacting with adaptor molecules , such as p62/sqstm1 , promotes uptake and degradation of both cargo and adaptors into the lysosomes .
indeed , autophagosomes fuse with lysosomes transporting intracellular proteins in the last autophagic steps . among them , also viral antigens can end up in the lysosomes , in virally infected cells . here
peptides derived from their degradation may be complexed and presented in association with class ii mhc molecules
. another important role of autophagosomes is to transport entire viral particles to the endosomal / lysosomal compartment , to be degraded and eventually eliminated , a process known as xenophagy . after entering the cells ,
alternatively , phagosomes as well as free viral particles can be engulfed by the double membrane vesicles of autophagosomes , before reaching the lysosomes .
the latter process is under the control of the atg genes , even if it represents a particular form of autophagy aimed at the antimicrobial defense .
in addition , autophagy facilitates the pathogen engagement of intracellular tlrs and the consequent release of cytokines , such as type i ifn .
the binding of tlrs as well as other molecules involved in the immune response , for example , cd46 , by ligands or pathogens that use them as receptors , also triggers autophagy .
this has been reported for epstein - barr virus ( ebv ) that binds tlrs   and for measles virus that uses cd46 as receptor .
intriguingly , since cd46 represents the cellular receptor also for human herpes - virus 6 ( hhv6 ) , it would be interesting to investigate the impact on autophagy of cd46 engagement by hhv6 . as if the role of autophagy in the immune response was not enough important , it has been recently reported that autophagy is induced by gm - csf in monocytes and that it stimulates monocyte differentiation into functional macrophages and dcs .
besides that , autophagy also plays an essential role in preventing apoptosis of these cells .
among the strategies that allow viruses to escape from the immune control , the impairment of monocyte differentiation into functional dcs represents a common one [ 1719 ] . based on the recent finding , suggesting that autophagy is involved in monocyte differentiation in macrophages and dcs , the interference with the autophagic process could represent one of the underlying mechanisms responsible for such impairment .
this strategy has been recently reported to be exploited by human hepatitis c virus ( hcv ) , whose infection of human monocytes results in a reduction of lysosomal cathepsins , in an autophagic block at the late steps , and , as a consequence , in an impairment of dc differentiation .
hcv - mediated reduction of lysosomal acidification , as result of cathepsin release , has been reported also in other cell types .
however , autophagy can contribute to hcv replication since the autophagosomal membranes can be used for viral production .
given the importance of autophagy in the immune response , it is not surprising that viruses have evolved strategies to interfere with it , in order to avoid their elimination into the lysosomes , impair the production of antiviral cytokines , reduce the presentation of their antigens , and , as described for hcv , alter dc differentiation .
this is a must for viruses to persist in the infected host , sometimes with pathological consequences .
all the steps of the autophagic process , from the autophagosome formation to the lysosomal degradation of their content , can be manipulated by viruses [ 24 , 25 ] ( figure 1 ) .
it has been described that it depends on the virus types , on the phase of their life cycle , and on the host cell that they infect .
for example , human immunodeficiency virus , a single - stranded rna lentivirus belonging to the retroviridae family , is able to induce the initial phases of autophagy by env , an envelope fusogenic protein . on the other hand ,
it blocks the late autophagic phases to enhance viral production by expressing nef , an accessory protein that interferes with the autophagosomal maturation [ 24 , 25 ] . during the replicative phase of their life cycle ,
also herpesviruses such as ebv and kaposi 's sarcoma human herpesvirus ( kshv ) can promote autophagy and exploit the autophagic machinery to enhance their replication [ 2628 ] .
to do so , they promote the first autophagic steps and block the last ones .
this strategy allows them to avoid being delivered into the destructive environment of the lysosomes and to usurp the autophagic machinery for viral transportation through the cell cytoplasm [ 22 , 26 , 29 ] .
pathways involved in the autophagy induction , such as pkr / eif2 alpha and m - tor , can be also targeted by viruses belonging to the herpesvirus family , for example , herpes simplex virus-1 ( hsv-1 ) [ 3032 ] and cytomegalovirus ( hcmv ) .
also beclin 1 , a key molecule involved in several steps of the autophagic process , can be bound and altered in its function by viral proteins such as vbcl2 of kshv or icp 34.5 of hsv-1 [ 3436 ] .
all these strategies allow viruses to block autophagy induction during infection of the host cells .
differently , another herpesvirus , varicella - zoster virus ( vzv ) , has been recently reported to successfully infect target cells without blocking the autophagic process .
indeed icp 34.5 or us11 , the two proteins responsible for the block of autophagy by hsv1 , are not present in vzv although both viruses belong to the same subfamily .
herpesviruses are large , double strand dna viruses having a common particle structure . to date eight human herpesviruses
have been identified and classified into three subfamilies ( alpha , beta , and gamma ) based on their growth characteristics and tissue tropism .
the -subfamily includes the neurotropic viruses herpes simplex viruses ( hsv ) 1 and 2 and varicella - zoster virus ( vzv ) .
human cytomegalovirus ( hcmv ) and the human herpesviruses 6 and 7 are members of the -subfamily , while ebv and kshv are members of the -subfamily . a common feature shared by all human herpesviruses
is the viral persistence into host and the possibility to undergo two alternative life cycle programs , namely , latency and lytic replication . during latency
the viral genome is retained as a circular episome in the nucleus and no viral progeny is produced .
furthermore , in the course of latent infection a limited set of genes is expressed in order to reduce immune recognition .
in contrast to latency , the herpesvirus lytic program is characterized by a regulated cascade of viral gene expression accompanied by viral production and killing of infected cells .
although most herpesviruses have been reported to interfere with the autophagic process , the next part of this review will focus on the interplay between autophagy and -herpesviruses , given that our laboratory has for long time worked on the virus - host interaction of these viruses associated with several human cancers .
as for other herpesviruses , ebv infection can be latent or lytic [ 41 , 42 ] . during latency
the ebv genome is retained as a circular episome in the cell nuclei and upon appropriate stimuli the virus switches into the lytic replication program .
it is known that ebv infects primarily human b lymphocytes and epithelial cells ; however , its infection can also occur in other cells with a central role in the immune response , such as monocytes and dcs [ 44 , 45 ] .
the in vitro infection of monocytes induces apoptosis and results in an impairment of dc development [ 45 , 46 ] , although the underlying mechanisms have not been investigated in these studies .
therefore , the autophagic pathway could be investigated in these cells since , as previously described , a block of autophagy can switch cell differentiation into cell death in monocytes dcs .
these are the most powerful cells in the priming of the cd8 t cell response , playing a pivotal role in control of the ebv infection . besides cytotoxic t cells , dcs are able to activate a cd4 t cell - mediated immune response against the ebv antigens , such as ebna1 , through the mechanism of cross - presentation .
autophagy has been shown to be essential for class ii mhc presentation of ebna1 protein .
the block of autophagy resulted in an accumulation of this protein in the intracellular autophagosomes and , more importantly , in a reduction of ebna1 recognition by ebna1 specific cd4 t cells .
the involvement of autophagy in ebna 1 presentation was later confirmed by a more recent study . regarding the ebv interaction with plasmacytoid subpopulation of dcs ( pdcs ) , the main type i ifn producing cells
, it has been reported that autophagy is essential for ifn release in response to the ebv infection .
autophagy is stimulated by the virus and facilitates its interaction with tlr7 and tlr9 pprs , both located in the endosomal / lysosomal compartment and essential for ebv recognition by these cells .
however , although ebv stimulates autophagy and ifn release , its infection results in an impairment of pdc maturation . since it has been reported that ebv downregulates tlr 9 in b cells   and that this effect is mediated by two ebv proteins , namely , latent membrane protein 1 ( lmp1 )   and bglf5 , it would be interesting to investigate tlr9 expression levels in the pdcs ebv - infected versus uninfected control cells .
ebv is associated with several different human cancers of b and epithelial cell origin , such as posttransplant lymphoproliferative disorder ( ptld ) , hodgkin and non - hodgkin lymphomas , nasopharyngeal carcinoma ( npc ) , and some forms of gastric carcinoma . besides its strong association with some human cancers , ebv infects and establishes a life - long asymptomatic infection in 95% of adult healthy population , reducing to the minimum or not expressing any protein , in order to escape from the immune recognition . among the viral antigens ,
only the ebv latent nuclear antigen 1 ( ebna1 ) expression is required for the maintenance of the viral episome in the ebv infected human b cells   and this is the only protein always expressed in ebv - associated malignancies .
this protein has been demonstrated to activate both cd4 t cells , being presented via class ii mhc   and cd8 t cells , although for long time it has been considered invisible to the immune system . as previously described , autophagy is essential for ebna1 antigen presentation via class ii mhc while , interestingly , it does not seem to play a role in the class ii mhc presentation of two other ebv latent nuclear proteins , ebna2 and ebna 3c , both expressed only in pathological conditions .
the most oncogenic ebv latent protein , lmp1 , has been reported to regulate its own expression by inducing or inhibiting autophagy .
when lmp1 is highly expressed in b cells , autophagy is stimulated and promotes degradation .
given that lmp1 plays an important role in ebv - induced oncogenesis , the stimulation of autophagy could be used as a strategy to reduce the expression of this protein and consequently to affect the ebv - driven tumorigenesis .
kshv or human herpesvirus-8 ( hhv-8 ) is the last human -herpesvirus identified to date .
generally , latency is the kshv default program within 4872  h postinfection depending on target cells . in vivo
, kshv mainly infects endothelial and b cells and establishes a lifelong latency in b lymphocytes of infected individuals , escaping from the host immune response [ 65 , 66 ] .
several kshv latent and lytic proteins are involved in immune evasion , in preventing apoptosis and blocking autophagy as well as in transformation .
kshv is the etiologic agent of kaposi 's sarcoma ( ks ) , a multifocal angioproliferative disorder arising from kshv - infected endothelial cell , and of lymphoproliferative disorders , such as primary effusion lymphomas ( pels ) , a non - hodgkin b cell lymphoma localized in body cavities , and the plasma cell variant of multicentric castleman disease ( mcd ) .
it is important to underline that , although kshv is detectable in all ks lesions , regardless of disease stage or clinical variant , the virus is necessary but not sufficient for the development of this tumor [ 67 , 68 ] .
the majority of spindle cells are latently infected by kshv and only a small percentage of these cells express viral lytic antigens [ 68 , 70 ] .
however , it is currently believed that lytic replication is necessary to support ks lesion formation and maintenance [ 68 , 7174 ] .
in addition , viral replication allows the secretion of proinflammatory and/or proangiogenic factors that create the inflammatory microenvironment essential in ks pathogenesis [ 68 , 75 ] .
monocytes are among the inflammatory cells found in ks lesions and a study , based on immunohistochemical staining and in situ hybridization , reported that these cells are kshv infected .
the virus has also been detected in peripheral t - cells and macrophages [ 77 , 78 ] .
as other herpesviruses , kshv has to cope with innate and adaptive immunity in order to establish a persistent infection in immunocompetent host .
latency is one of the strategies used by kshv since viral gene expression is reduced .
conversely , during its lytic cycle , a high number of immunogenic proteins are produced [ 71 , 79 ] .
the relevance of the equilibrium between kshv lytic cycle and host immunity is indicated by the increased viral replication and the development of kshv - related malignancies in immunosuppressed patients [ 8082 ] . about 25% of kshv proteins
are involved in kshv immune evasion mechanisms   and , although most of these belong to lytic cycle , both latent and lytic proteins are able to hijack innate and adaptive immune response .
they can indeed inhibit complement - mediated lysis of infected cells and the ifn i signaling , deregulate the inflammatory cytokine / chemokine networks , and interfere with antigen presentation [ 67 , 79 , 8385 ] .
pdcs , monocyte - derived dendritic cells , and monocytes are among the cell types infected by kshv [ 19 , 86 , 87 ] . as consequence of the infection , a reduction of costimulatory molecules as well as a deregulation of the cytokine release and an impairment of allostimulatory capacity can occur [ 19 , 76 , 8790 ] . in vivo , dc functional impairment has been reported in patients with classical ks   and a reduction of pdcs was observed in aids - ks in comparison to ks negative hiv-1 infected individuals .
moreover , the number of langerhans cells is also decreased in ks lesions compared to normal skin .
furthermore , it has been reported that the signal transducer and activator of transcription 3 ( stat3 ) activation in noninfected monocytes / macrophages leads to a block of autophagy and consequent dysfunction , due to the cytokines released during hiv infection .
we showed that khsv is able to activate stat3 pathway in dcs by binding to its receptor on these cells , namely , the type ii c - type lectin , dendritic cell - specific icam-3 grabbing nonintegrin ( dc - sign ; cd209 ) [ 87 , 96 ] . besides , we showed that stat3 activation led to a block of the autophagic flux , as demonstrated by the reduced expression of lc3ii and the increased level of p62 .
in addition , looking for a possible mechanism responsible for the block of autophagy mediated by stat3 activation in dcs exposed to kshv , we found an upregulation of mcl-1 [ 96 , 97 ] .
this is one of the proteins able to bind and sequester beclin 1 , hampering its essential role in autophagosome formation .
remarkably , stat3 inhibition by ag490 was able to prevent the effects on p62 , lc3ii , and mcl-1 . in agreement
, a previous paper has shown that the inhibition of stat3 by sorafenib resulted in a downregulation of mcl-1 , disruption of beclin 1-mcl-1 complex , and reversion of the autophagic block in hepatocarcinoma cells .
of note , the autophagic block occurred also in the presence of uv - inactivated kshv , indicating that neither vbcl2 nor vflip expression was required [ 34 , 96 , 99 , 100 ] .
concomitantly to the autophagic block induced by kshv - mediated stat3 activation , we also observed a reduction of il12p70 release in response to lps stimulation and a higher production of il-10 , il-6 , and il-23 .
this cytokine pattern skews the th1/th2 profile towards th2   and/or th17 , promoting immunosuppression and inflammation .
therefore , stat3 activation could be one of the molecular mechanisms underlying kshv - mediated immunosuppression in dcs .
indeed , stat3 activation correlates with an immunosuppressive phenotype and dc dysfunction in the ks microenvironment   and in the peripheral blood of tumor bearing patients [ 103 , 104 ] .
several papers have investigated the interplay between kshv and the autophagic pathway . during the latent phase of its life - cycle
, kshv expresses the fadd - like interleukin-1 beta - converting enzyme inhibitory protein ( v - flip ) , a truncated homolog of the cellular flip , which besides having an antiapoptotic activity [ 105 , 106 ] has been recently shown to block the autophagic flux .
it competes with lc3 for binding to atg3 , thereby preventing lc3 binding and processing during autophagosome biogenesis .
furthermore , the inhibition of vflip binding to atg3 reduced the size of kshv positive tumor in mice .
it has also been demonstrated that v - flip suppression of autophagy counteracts v - cyclin - induced cellular senescence [ 107 , 108 ] .
v - cyclin is a kshv latent protein that deregulates cell cycle , causes aberrant host dna replication , and triggers the dna damage responses ( ddrs ) . besides vflip , two kshv proteins expressed during the lytic cycle , vbcl2 and k7 , can interfere with autophagy .
vbcl2 is a homolog of cellular bcl-2 that inhibits both apoptosis [ 99 , 110 ] and autophagy [ 34 , 35 ] . as its cellular counterpart
, vbcl2 negatively regulates autophagy by binding beclin 1 . always within the context of the lytic cycle
, recent evidence suggests that kshv k7 protein prevents autophagosome maturation , by interacting with rubicon autophagic protein .
k7 is involved in apoptosis suppression [ 112 , 113 ] while rubicon is a subunit of the beclin 1/uvrag / vps34 autophagy complex , which regulates the autophagosome maturation and the endocytic trafficking [ 114116 ] .
as shown by authors , k7 transfection in epithelial cell lines promoted a greater interaction between rubicon and beclin 1/uvrag / vps34 complex resulting in a block of vps34 enzymatic activity and , consequently , of autophagosome maturation .
interestingly , during the lytic cycle kshv exploits autophagy to enhance its reactivation mediated by rta immediate lytic protein .
altogether , the data here reviewed suggest that kshv has evolved several strategies to circumvent autophagy - mediated immune responses , to persist in infected hosts .
moreover , the autophagic block during latency and the autophagy induction during the lytic cycle may contribute to the pathogenesis of kshv associated malignancies .
a better understanding of how autophagy manipulation could influence antiviral immune response and control virus - induced tumorigenesis might help to discover strategies improving the outcome of the treatments of virus - associated malignancies .
moreover , since autophagy is activated and is involved in ebv and kshv replication [ 20 , 27 ] , its manipulation could also affect the viral particle release that plays a role in -herpesvirus - associated cancers .
 we summarized the most recent findings on the role of autophagy in antiviral immune response . 
 we described how viruses have developed strategies to subvert the autophagic process . 
 a particular attention has been given to epstein - barr and kaposi 's sarcoma associated herpesvirus , viruses studied for many years in our laboratory . 
 these two viruses belong to -herpesvirus subfamily and are associated with several human cancers . besides the effects on the immune response , 
 we have described how autophagy subversion by viruses may also concur to the enhancement of their replication and to viral tumorigenesis . 