the immunosuppressive receptor pd-1 and its ligand pd - l1 have been identified by dr . tasuku honjo and his colleagues at the kyoto university as factors that 
 induce programmed cell death . since the mechanism mediated by pd-1 and its ligand was shown to be important for immune 
 suppression and tolerance , it has also been reported to be involved in pathogenetic mechanisms of various diseases . pd-1 and pd - l1 are single - pass transmembrane molecules expressed on the cell surface , which belong to the b7 family . their expression is induced by 
 immune - activating stimuli and is understood as a negative feedback mechanism to suppress excessive immune reactions and let immune responses cease . when the 
 receptor pd-1 is bound by its ligand , src homology 2-domain - containing tyrosine phosphatase 2 ( shp-2 ) and shp-1 are recruited to immunoreceptor tyrosine - based 
 inhibitory motif and immunoreceptor tyrosine - based switch motif in the intracellular region of pd-1 and suppress antigen receptor signaling mediators . this process results in reduced production of cytokines , such as interferon ( ifn)- and interleukin ( il)-2 , reduced cell 
 proliferation and suppressed immune cell activation . in addition to the close involvement in homeostasis of the living body as summarized above , recent studies 
 have shown that immunosuppressive factors , pd-1 and pd - l1 , are also involved in immunosuppression in cases of tumors and chronic infections . while generally not 
 highly expressed in normal tissues , pd - l1 is expressed at a high rate in tumor tissues in melanoma , lung cancer , colorectal cancer and ovarian cancer , and has 
 been demonstrated to be involved in immune evasion by tumors . in patients with renal cell cancer and gastric cancer , 
 pd - l1 has been shown to be an important prognostic determinant ; also , the disease progression is faster , and the mortality is higher in patients with tumors 
 expressing pd - l1 than in patients without detectable pd - l1 expression [ 51 , 53 ] . in 
 addition , pd - l1 expression has also been reported for other tumors including breast cancer , pancreatic cancer and bladder cancer , and pd-1 expression in 
 tumor - infiltrating lymphocytes has been confirmed in many types of tumors including melanoma , lung cancer and intrahepatic bile duct cancer . these findings have 
 revealed the importance of the pd-1/pd - l1 pathway as an immunosuppression mechanism in a variety of tumors . based on the series of studies on pd-1/pd - l1 in tumor 
 diseases , therapeutic means targeting this pathway are being developed . specifically , these new therapeutics are biopharmaceuticals based on anti - pd-1 antibodies , 
 anti - pd - l1 antibodies or recombinant proteins that inhibit the pd-1/pd - l1 pathway . these biopharmaceuticals have been reported to show good anti - tumor effects 
 regardless of the tumor type and are attracting growing attention as a new , promising class of anti - tumor therapy . fully humanized anti - pd-1 and anti - pd - l1 
 antibodies have already been produced and are actively tested as therapeutic agents in clinical studies ( trials ) , with good anti - tumor effects continuing to be 
 reported in patients with melanoma , lung cancer , renal cell cancer and some other types of cancer . in melanoma , which has a very poor prognosis , clinical trials 
 conducted in japan and the united states have reported not only the suppressed growth of cancer cells , but also complete remission in some cases . on september , 
 2014 , the anti - pd-1 antibody was finally released by ono pharmaceutical as a new anti - tumor therapeutic agent , and it has become a pioneer 
 for innovative immune checkpoint inhibitors ( ono pharmaceutical : https://www.opdivo.jp/contents/action/ ) . this immune checkpoint - targeted immunotherapy was 
 selected as  breakthrough of the year 2013  by the journal science as a global revolutionary technology . currently , merck , roche and other major pharmaceutical 
 companies around the world are accelerating their efforts to develop similar antibody - based drugs ; this class of therapeutics is gaining so much momentum and 
 attention that the conventional concept of anti - tumor therapy is being overturned . the causative factors of bovine leukemia can be divided 
 into viral and non - viral . non - viral bovine leukemia is sporadic and can be subdivided into calf type , thymic type and cutaneous type involving unknown causes . on 
 the other hand , enzootic bovine leukemia , which is caused by blv , accounts for the vast majority of cases of bovine leukemia , and its prevalence continues to 
 increase . blv infections are latent in the aleukemic ( al ) state , but can emerge as persistent lymphocystosis ( pl ) with non - malignant polyclonal expansion of 
 cd5 b - cells that predominantly harbor blv provirus and rarely as malignant b - cell lymphoma in various lymph nodes after long periods of latency 
 . the progression of enzootic bovine leukemia is accompanied by marked suppression of cell - mediated immunity [ 8 , 9 , 16 ] , and as the pathogenetic mechanism 
 remains unknown , there are no effective vaccines or therapeutic methods available , meaning that affected animals eventually die . bovine leukemia was designated as 
 a communicable disease obligated to notify under the act on domestic animal infectious diseases control when it was revised in 1997 . in 2015 , 2,896 cases of 
 bovine leukemia were reported ( of which the largest number of 494 cases occurred in hokkaido ) , representing the disease reported in a greater number than any 
 other bovine disease required to monitor by the act ( http://www.maff.go.jp/j/syouan/douei/kansi_densen/kansi_densen.html ) . this number is 29.3-fold the number of 
 cases reported in 1998 ( 96 affected animals ) , indicating that the increase has not yet been halted . requests for urgent measures against this disease have been 
 voiced very frequently by people practicing veterinary medicine and animal husbandry . however , a large - scale survey conducted by the national institute of animal 
 health using specimens collected from 2009 to 2011 showed an approximate blv - positive rate of 35% , demonstrating the difficulty of implementing any project to 
 select and replace infected cattle . comparative analyses of blv - infected cattle by clinical condition have suggested that animals with a high viral load and persistent lymphocytosis are at a high 
 risk of disease onset , often serving as infection sources , have a high risk of vertical transmission   and have increased levels of cd4cd25foxp3 treg cells , showing increased susceptibility 
 to opportunistic infections due to transforming growth factor - beta produced by treg cells , which reduces the production of interferon - gamma and tumor necrosis 
 factor - alpha by cd4 t cells and impairs cellular immunity mediated mainly by the cytotoxic activity of nk cells [ 28 , 48 ] . moreover , the proliferative ability of lymphocytes in response to blv was also significantly reduced in 
 cattle with persistent lymphocytosis , and lymphocytes were found to produce reduced levels of anti - viral cytokines , such as ifn- , il-2 and il-12 [ 13 , 18 ] . therefore , we analyzed the expression of pd-1 and its ligand , pd - l1 . results 
 showed that the pd-1 expression in cd4 and cd8 cells and the pd - l1 expression in virus - infected b cells increase as the disease 
 progresses . furthermore , pd - l1 expression negatively correlated with the ifn- expression level , an indicator of immunosuppression , while positively correlating 
 with leukocyte count , virus titer and provirus level . antibodies to bovine pd-1 and pd - l1 were established , and the results from pd-1 and pd - l1 binding inhibition 
 assays confirmed that they activated anti - viral immunity and that the increase in ifn- production positively correlated with the pd-1 expression rate on 
 cd4 t cells . and also , recombinant bovine pd - l2 ( pd - l2-ig ) significantly enhanced ifn- production from virus antigens - stimulated pbmcs derived from 
 blv - infected cattle . interestingly , the pd - l2-ig - induced ifn- production was further enhanced by treatment with anti - bovine pd-1 antibody . these results indicated that the pd-1/pd - l pathway constitutes a part of the immunosuppressive mechanism in bovine leukemia ( table 2table 2.change of immune inhibitory molecules in the cause of bovine leukemia virus infectionreceptor / liganduninfectedinfected(disease status)referencesalpllymphomapd-1/pd - l1///// lag-3/mhcclass ii////n.d [ 17 , 41]tim-3/gal-9///// ctla-4/cd80 , cd86/n.d/n.d/n.d/n.dn.d n.d : not demonstrated . ) we also analyzed immunosuppressive receptors other than the pd-1/pd - l1 pathway , 
 such as lymphocyte - activation gene 3 ( lag-3 ) [ 17 , 41 ] , t - cell immunoglobulin and 
 mucin domain - containing protein 3 ( tim-3 ) , and cytotoxic t - lymphocyte antigen 4 ( ctla-4 ; cd152 )   expressed in antigen - specific lymphocytes and found that expression levels of lag-3 , tim-3 , ctla-4 and their ligands on lymphocytes 
 increased as the disease progressed and anti - viral immunity was activated in binding inhibition assays , as was the case for pd-1 ( table 2 ) . currently , clinical studies of these drugs are being carried out at the hokkaido university and other institutions . although there are many diseases in cattle involving immune abnormalities ( impairments ) , the mechanisms underlying these diseases remain unknown . our previous 
 analyses have shown that immunosuppressive factors , such as pd - l1 , are also involved in immune suppression seen in diseases other than bovine leukemia , namely 
 johne s disease   and bovine anaplasmosis . furthermore , recent evidence 
 suggests that immunosuppressive factors , such as pd-1 , are involved in the reduced immune function in chronic infectious diseases , such as mastitis , bovine 
 mycoplasmosis and bovine tuberculosis ( manuscript in preparation ) . our laboratory has been working on the development of a blv vaccine for many years . it is based 
 on a vaccine antigen that was found to be promising in in vitro studies . however , although we tried various procedures , the vaccine did not 
 prevent infection or even disease onset despite the fact that effector cells were present in vivo ( data not shown ) . results from the present 
 analysis suggested that the virus s immune evasion mechanism for lymphocyte exhaustion might be related to the ineffectiveness of the vaccine . future measures 
 against chronic infectious diseases will require the development of a new , pre - emptive control method that targets this formidable immune evasion mechanism . to 
 achieve this goal , results from more detailed analyses of immune exhaustion in other chronic infections are awaited . for use in humans , several immune 
 checkpoint - targeting biopharmaceuticals have been successively developed , including those described above , and they are being actively tested in clinical trials . 
 in the future , it is anticipated that they will be applied to veterinary medicine and animal husbandry , including diseases in cattle .
recently , dysfunction of antigen - specific t cells is well documented as t - cell exhaustion and has been defined by the loss of effector functions during chronic 
 infections and cancer in human . 
 the exhausted t cells are characterized phenotypically by the surface expression of immunoinhibitory receptors , such as 
 programmed death 1 ( pd-1 ) , lymphocyte activation gene 3 ( lag-3 ) , t - cell immunoglobulin and mucin domain - containing protein 3 ( tim-3 ) and cytotoxic t - lymphocyte 
 antigen 4 ( ctla-4 ) . 
 however , there is still a fundamental lack of knowledge about the immunoinhibitory receptors in the fields of veterinary medicine . in 
 particular , very little is known about mechanism of t cell dysfunction in chronic infection in cattle . 
 recent our studies have revealed that immunoinhibitory 
 molecules including pd-1/ programmed death - ligand 1 ( pd - l1 ) play critical roles in immune exhaustion and disease progression in case of bovine leukemia virus 
 ( blv ) infection , johne s disease and bovine anaplasmosis . 
 this review includes some recent data from us .