Efficacy of Immune Checkpoint Blockade and Biomarkers of Response in Lymphoma: A Narrative Review

Immune checkpoint blockade (ICB) has revolutionized the prognosis of several advanced-stage solid tumors. However, its success has been far more limited in hematological malignancies and is mostly restricted to classical Hodgkin lymphoma (cHL) and primary mediastinal B cell lymphoma (PMBCL). In patients with non-Hodgkin lymphoma (NHL), response to PD-1/PD-L1 ICB monotherapy has been relatively limited, although some subtypes are more sensitive than others. Numerous predictive biomarkers have been investigated in solid malignancies, such as PD-L1 expression, tumor mutational burden (TMB) and microsatellite instability (MSI), among others. This review aims to appraise the current knowledge on PD-1/PD-L1 ICB efficacy in lymphoma when used either as monotherapy or combined with other agents, and describes potential biomarkers of response in this specific setting.


Introduction
The tumor microenvironment (TME), consisting of T cells, tumor-associated macrophages (TAMs), dendritic cells (DCs), neutrophils and natural killer (NK) and stromal cells, is thought to play a significant role in the development and progression of many cancers, as well as in tumor escape from the immune system. Accumulation of somatic mutations during oncogenesis has been shown to result in the presentation of neoantigens at the tumor cell surface, which can elicit tumor-specific CD4+ and CD8+ T cells with antitumor potential [1]. Many studies have demonstrated that tumor evasion is, at least in part, mediated by the inhibition of antitumor T-cell responses, mostly via upregulation of immune checkpoint molecules [2][3][4]. The understanding of these resistance mechanisms led to the delineation of the concept of immune checkpoint blockade (ICB).
More precisely, interaction between the immune checkpoint (IC) molecule programmed cell death protein 1 (PD-1) expressed by activated tumor-infiltrating T cells and its ligands (PD-L1 and PD-L2) expressed by the surrounding tumor and TME cells commonly leads to downregulation of neoantigen-specific T-cell responses. Blocking these interactions is a frequently used therapeutic approach to restore the antitumor effect of the host immune system [5,6].
In lymphoproliferative disorders, PD-1 is frequently expressed on tumor cells themselves as in tumor-infiltrating lymphocytes (TILs), while its ligands may be upregulated by tumor cells (some B-cell or T-cell lymphomas) but also TME cells such as TAMs, mast cells and mesenchymal cells [7][8][9]. PD-1 blockade using nivolumab or pembrolizumab has dramatically improved the prognosis of relapsed/refractory (R/R) classical Hodgkin lymphoma (cHL) and is now a well-recognized therapeutic option in this setting [10][11][12]. On the other hand, the efficacy of PD-1 or PD-L1 blockades in non-Hodgkin lymphoma (NHL) has shown disappointingly low response rates, except for some specific subsets of NHL, such as primary mediastinal B cell lymphoma (PMBCL) [13], primary testicular lymphoma (PTL) or primary central nervous system lymphoma (PCNSL) [14,15]. Interestingly, these NHL subsets have been shown to be highly infiltrated by T cells [16,17]. Of note, the different anti-PD-1/PD-L1 antibodies used in the clinic vary in their IgG isotypes and affinity to the various Fc gamma receptors (FcγRs) expressed on immune cells (Table 1); however, whether these changes translate into different clinical benefits is not well established [18]. A persistent challenge remains the identification of predictive biomarkers of response to PD-1/PD-L1 ICB in this setting. Here, we present a review of the clinical efficacy of PD-1/PD-L1 ICB as monotherapy and in combination with other agents for different lymphoma subtypes. The safety and activity of PD-1 ICB was first tested by  in a phase 1 study on 23 heavily pretreated cHL patients who received biweekly nivolumab, a PD-1 inhibitor, demonstrating an impressive 87% overall response rate (ORR) and a 17% complete metabolic response rate (CMR) [11,29]. Larger studies subsequently confirmed the efficacy of PD-1 ICB monotherapy in R/R cHL using either nivolumab or pembrolizumab (Table 2) [21,[30][31][32]. All trials showed similar results in R/R cHL patients, including those relapsing after autologous stem cell transplantation (ASCT) and those ineligible for ASCT (Table 2). At a 5-year follow-up, both CheckMate 205 and KEYNOTE-087 trials demonstrated sustained responses, with median PFSs of 15 months and 13.7 months, respectively. Additionally, a subgroup analysis of the KEYNOTE-087 trial demonstrated the efficacy of pembrolizumab after treatment with the anti-CD30 antibody drug conjugate brentuximab vedotin (BV) ( Table 2) [33]. Based on these results, the FDA approved nivolumab (2016) and pembrolizumab (2017) for R/R cHL patients relapsing after ASCT and BV [34]. More recently, the phase 3 KEYNOTE-204 study (2021) compared pembrolizumab and BV in patients with R/R cHL either following ASCT or in patients ineligible for ASCT and showed an increased median progression-free survival (PFS) for pembrolizumab over BV (Table 2) [35].      In addition to nivolumab and pembrolizumab, several other PD-1 inhibitors have been investigated in R/R cHL early-phase trials. PD-1/PD-L1 antibodies vary in their IgG isotypes (Table 1). PD-1 IgG4 antibodies include nivolumab, pembrolizumab, sintilimab, camrelizumab, tislelizumab and zimberelimab, with ORRs ranging from 42% to 91% [24][25][26][27][40][41][42][43][44], while penpulimab, an IgG1 antibody, demonstrated an ORR of 89% with a 47% CR in a multicenter phase 1/2 trial [28]. To our knowledge, avelumab, an IgG1 anti-PD-L1 antibody, is the only anti-PD-L1 antibody that has been tested in this setting, achieving a 42% ORR and a 19% CR [39] (Table 2).

PD-1/PD-L1 ICB in Combination with Chemotherapy Frontline Setting
Assuming that PD-1/PD-L1 ICB may prime the TME for the induction of antitumor T-cell responses before patients receive cytotoxic agents, both sequential and concomitant combinations of ICB with conventional chemotherapy have been evaluated in the frontline setting. Ramchandren et al. (2019) evaluated a sequential approach of four cycles of single-agent nivolumab followed by twelve cycles of nivolumab given concomitantly with doxorubicin, vinblastine and dacarbazine (N-AVD) in patients with newly diagnosed advanced-stage cHL (cohort D of CheckMate 205) ( Table 3) [61]. N-AVD has also been investigated in early-stage unfavorable cHL in a phase 2 trial conducted by the German Hodgkin Study Group (2020), achieving similar response rates (Table 3) [62]. N-AVD regimen is currently being evaluated in a frontline randomized phase 3 study against BV-AVD in patients with advanced-stage cHL, with an estimated completion date in 2024 (NCT03907488). Similarly, a sequential strategy of pembrolizumab monotherapy followed by combination with AVD has been evaluated in unfavorable or advanced-stage cHL patients (Table 3) [63]. Another ongoing nonrandomized PET-adapted phase 2 study (NCT03617666) is investigating the safety and efficacy of sequential avelumab followed by ABVD in first-line high-risk cHL [64]. As approximatively twenty percent of newly diagnosed cHL patients are ineligible for intensive chemotherapy regimens and consequently at risk of experiencing worse outcomes [65,66], Cheson and colleagues (2020) investigated the combination of 8 cycles of BV with nivolumab for older (>60 years) or chemo-ineligible newly diagnosed cHL patients. The trial was prematurely closed after an interim analysis failed to show that the combination met the predefined ORR criteria (ORR > 68%); however, 61% of all evaluable patients displayed an objective response and 48% achieved CMRs, demonstrating that this well-tolerated combination is active in this frail population (Table 3) [67].

Relapsed/Refractory Setting
Herrera and colleagues (2018) evaluated a nivolumab and BV combination for up to four cycles in the first-salvage-setting phase 1/2 study; this regimen was demonstrated to be well tolerated, achieving an ORR of 82%, including a 61% CMR [75]. An extended 3-year follow-up confirmed the durability of responses, with an estimated 77% 3-year PFS after a median follow-up of 34.3 months [76]. A phase 3 trial further investigating this regimen versus BV alone in R/R or ASCT-ineligible cHL patients was closed due to insufficient enrolment (NCT03138499). Three other trials demonstrated the efficacy of PD-1 ICB combined with standard salvage chemotherapy (ICE or gemcitabine, vinorelbine and liposomal doxorubicin (GVD) prior to ASCT) ( Table 3) [68][69][70]77]. Atezolizumab, a PD-L1 inhibitor, scarcely tested as monotherapy in lymphoma ( [59], NCT03120676), is currently being investigated in combination with a BeGEV regimen (bendamustine, gemcitabine and vinorelbine) (NCT05300282) ( Table 3).
Patients with R/R HL who undergo ASCT have an expected 60% 18-month PFS [112][113][114]. Lepik et al. evaluated the utility of nivolumab and bendamustine administered for up to three cycles in heavily pretreated R/R cHL patients who had failed at least two lines of prior therapy, including nivolumab monotherapy. Among all enrolled patients (n = 30), 26 achieved a response (ORR: 87%) and 17 a CMR (57%) (
Various combinations of PD-1/PD-L1 ICB with novel immunotherapies are currently under investigation in early-phase trials. A first report of pembrolizumab combined with AFM-13, a CD30-CD16 bispecific antibody stimulating innate immune cells, such as NK and macrophages, achieved an ORR of 83% in R/R cHL patients who had received a median of three prior lines of therapy [78,79]. A recent phase 1 study evaluating the benefit of adoptive cellular therapy consisting of tumor-associated antigen (TAA)-specific T cells enrolled 10 patients with R/R HL (n = 8 active disease, n = 2 adjuvant after ASCT) to receive TAA-specific T cells (autologous or allogenic) with nivolumab given as a priming agent in 6 out of the 10 patients. Among the patients with active disease, one patient achieved CMR and seven had stable disease (SD) at 3 months [78]. In addition, there is growing evidence that chimeric antigen receptor (CAR) T-cell fitness may be improved by the adjunction of PD-1 ICB. With the limitation of a small sample size (n = 12), PD-1 ICB administration after CD30 CAR T-cell therapy in CD30+ lymphoma patients (n = 9 cHL, n = 1 angioimmunoblastic T-cell lymphoma, n = 2 gray zone lymphoma) suggested improved efficacy (ORR 86% versus 100%; CR 27% versus 80%) ( Table 3) [111]. Similarly, the role of anti-PD-1 therapy after CD30 CAR T-cell treatment is currently being evaluated in R/R cHL patients (NCT04134325) [115,116]. Recently, Timmerman et al. (2022) demonstrated that the association of favezelimab (lymphocyte activating gene-3 (LAG-3) ICB) and pembrolizumab could be an effective therapeutic option for patients progressing under PD-1 ICB (ORR 31%, CR 7%) [82]. Other combination approaches in the R/R cHL setting are ongoing, such as nivolumab with ruxolitinib (NCT03681561) and nivolumab/pembrolizumab combined with radiation (NCT04419441). First tested on a range of various hematological malignancies, nivolumab achieved an ORR of 36% and a CMR of 18% in the 11 R/R diffuse large B cell lymphoma (DLBCL) patients enrolled in a phase 1 study [53]. Single-agent activity was further evaluated in DLBCL patients who were relapsing after ASCT or ineligible for ASCT [49] and in patients with various hematologic malignancies relapsing after allogeneic stem cell transplantation (allo-SCT) [55], demonstrating modest benefits ( Table 2). In contrast to cHL, maintenance treatment with pembrolizumab administered after ASCT in DLBCL and PMBCL patients did not show any clinical benefit [54]. A randomized phase 3 trial investigating tislelizumab, a PD-1 inhibitor, as maintenance in DLBCL after ASCT is planned (NCT04799314). Even though the benefit of PD-1/PD-L1 inhibitors as monotherapy in R/R aggressive NHL has been disappointing, better activity has been observed in patient subsets, such as PMBCL, PCNSL and PTL patients [14,15,53]. Benefits from pembrolizumab were demonstrated in R/R PMBCL with 45-48% ORR and 13-33% CMR, leading to the accelerated approval of pembrolizumab by the FDA in this setting (Table 2) [32,117]. Nivolumab demonstrated activity in four patients with R/R PCNSL and one patient with PTL CNS recurrence; all five patients had clinical and radiographic responses to the monotherapy [15]. The efficacy in these specific aggressive NHL subgroups is likely due to their particular biology. This is described in further detail in the biomarker section. Pembrolizumab has also been tested on nine patients experiencing Richter transformation (RT) ( Table 2) [56].

Non-Hodgkin
PD-1/PD-L1 ICB demonstrated modest results in natural killer NK/T-cell NHL. In a phase 1 trial, nivolumab monotherapy achieved no observed CR (Table 2) [53]. On the other hand, pembrolizumab showed slightly improved activity in cutaneous T-cell lymphoma (CTCL) [57] and in a series of seven R/R extranodal NK/T-cell lymphoma (ENKTL) patients, with two of them achieving CR [118]. This initial signal of activity in R/R ENKTL was recently confirmed by two phase 2 trials evaluating avelumab, a PD-L1 inhibitor, and sintilimab, a PD-1 inhibitor [51,52] (Table 2).

PD-1/PD-L1 ICB in Combination with Other Agents Aggressive NHL
Frontline setting PD-1/PD-L1 ICB does not seem to add much benefit to frontline immunochemotherapy in newly diagnosed DLBCL. The combination of atezolizumab with frontline R-CHOP was tested on DLBCL patients [89,90] as a combination of pembrolizumab with R-CHOP in patients with either DLBCL or grade 3b FL (Table 3) [91]. Similarly, a durvalumab (another PD-L1 inhibitor) and R-CHOP combination did not seem to add any benefit (Table 3) [92].
In R/R PMBCL patients, the phase 2 CheckMate 436 trial (2019, 2021) evaluated nivolumab with BV (n = 30), reporting an ORR of 73% and a 37% CMR. Responses were durable at extended follow-up with a 2-year PFS of 56% (Table 3) [98,99]. A phase 1 study testing the addition of nivolumab and lenalidomide to immunochemotherapy in PCNSL is now recruiting (NCT04609046). In advanced ENKTL, a combined sintilimab, pegasparaginase, gemcitabine and oxaliplatin regimen has demonstrated an acceptable toxicity profile and promising efficacy in a phase 1 study [119]; a phase 2 trial is currently ongoing (NCT04127227).
With the exceptions of specific NHL subtypes and potentially CAR T-cell association, PD-1/PD-L1 ICBs generally do not add much clinical benefit when combined with other active agents in the R/R NHL setting.

Relapse/refractory setting
The addition of PD-1/PD-L1 ICB to anti-CD 20 monoclonal antibodies has been extensively tested in the R/R setting of indolent lymphomas, with no or only modest results (Table 3) [85][86][87]101]. Atezolizumab with obinutuzumab and lenalidomide resulted in a 78% ORR and a 72% CR rate [88]. The LYSA group investigated atezolizumab, obinutuzumab and venetoclax in R/R FL and MZL with reported ORRs of 54% and 67%, respectively [105]. Durvalumab or nivolumab in combination with ibrutinib resulted in modest response rates in R/R FL patients in two trials (Table 3) [102,103,108].

Predictive Biomarkers of Response to ICB in Lymphoma
Predictive biomarkers of response to ICBs have been identified in different solid tumors, but our ability to accurately predict response in lymphoma remains suboptimal. Mechanisms of immune evasion may differ from one lymphoma subtype to another. Several biomarkers have been investigated in lymphoid malignancies, including the PD-L1 Hscore, alterations/amplification of the 9p24.1 gene, microsatellite instability (MSI), tumor mutational burden (TMB), the density of intratumoral CD8+ T lymphocyte infiltrates, genetic alterations in MHC classes I and II, and miRNA-21, among others [120].

Tissue Expression and Plasma Levels of PD-Ls
PD-L1 and PD-L2 expression of TME can be influenced by two main signaling pathways. The extrinsic pathway relies on the release of inflammatory signals (i.e., IFN-γ) by TILs after tumor antigen recognition, consequently upregulating the expression of PD-L1/PD-L2 in tumor cells and TME cells [121]. On the other hand, the intrinsic pathway is mainly driven by genomic alteration of the 9p24.1 gene, EBV infection and the activation of the JAK/STAT transcription pathway; this is reviewed below [13,122].

PD-L1 Expression
Different scoring systems are used to determine PD-L1 expression (measured using immunohistochemistry; IHC) in tumors; however, its discriminative value is likely imperfect as patients with PD-L1-negative tumors may achieve durable responses [123]. Additionally, the PD-L1-positive cut-off threshold remains highly variable depending on tumor type, with different scoring systems used to determine PD-L1 expression in tumors [124]. The main scores are the combined positive score (CPS) [125], the tumor proportion score (TPS) [126] and the H-score [120]; the latter is the most-used scoring system for lymphoma [120].
Even though some studies revealed that TME PD-L1 expression correlates with poor prognosis in HL and NHL [127], this does not seem to be true for all lymphoma subtypes (i.e., NK/T lymphoma) [128]. Similarly, PD-L1 expression is not a reliable biomarker of response to PD-1/PD-L1 ICB for all lymphomas. For reasons explained below (refer to section), PD-L1 expression correlates to the response to anti-PD-1 therapy in cHL and some non-GCB DLBCL subtypes, but has no predictive value for other subtypes [128,129]. Similarly, soluble PD-L1 (sPD-L1), corresponding to plasma levels of PD-L1, has been investigated as a potential biomarker, with mixed results [130][131][132].

PD-L2 Expression
The prognostic significance of PD-L2 expression in malignant tumors remains controversial [133,134]. Gu and colleagues demonstrated that PD-L2 expression on DLBCL cells was significantly associated with prolonged OS and PFS. Higher ORRs to R-CHOP/CHOP were also reported in patients with enhanced expression of PD-L2 in malignant and immune cells [135]. Similarly, Tobin et al. demonstrated that PD-L2 is mostly present in the TME of FL and that a low PD-L2 expression was associated with worse outcomes [136].

Gene Alterations to 9p24.1
Copy number alterations (CNAs) to chromosome 9p24.1 (i.e., polysomy, copy gain, amplification, rarely translocation), leading to increased expression of PD-1 ligands in cHL, PMBCL and some extranodal large B-cell NHLs, are an important mechanism of tumor immune evasion. These alterations have been reported in the majority of cHLs (97%) [135], 63-75% of PMBCLs [137,138], over 40% of PTLs and roughly 20% of PCNSLs [139]. Unlike cHL, there is generally a low incidence (10-27%) of structural variations in PD-L1/PD-L2 in DLBCL [140,141]. The H-score could be used as a surrogate marker of the level of 9p24.1 gene alteration and predict the response to ICB [142]. cHL patients presenting 9p24.1 CNAs are more likely to present with advanced-stage disease and worse prognosis, with significantly shorter PFS [61]. On the other hand, they also tend to benefit from therapy with a PD-1 inhibitor [61,135,143]. However, Green et al. (2010) described that Hodgkin Reed-Sternberg cell lines harboring low levels of 9p24.1 CNAs still expressed PD-L1, suggesting that PD-L1 expression could be driven by other mechanisms [135]. Although the vast majority of NHLs have a considerably low sensitivity to PD-1/PD-L1 ICB, particular subtypes, such as PCNSL, PTL and PMBCL harbor 9p24.1 CNAs, conferring them an increased vulnerability to these agents [15,139]. In PMBCL, the magnitude of 9p24.1 CNAs is significantly associated with PD-L1 expression and survival outcome [14].
Even though the underlying mechanisms have not yet been fully elucidated, recent studies indicate that viruses also use the PD-1 signaling pathway to escape immune detection [145]. Thus, EBV-positive lymphomas tend to benefit from ICB therapy [146,147]. The expression of EBV latent membrane protein 1 (LMP1) or latent membrane protein 2a (LMP2a) was shown to be sufficient to activate the signaling cascade of the JAK/STAT pathway, leading to PD-L1 overexpression on tumor cells [145]. The expression of EBV has been implicated in approximately 40% of cHLs, 50-70% of post-transplant lymphoproliferative disorders (PTLDs), more than 95% of endemic Burkitt lymphomas (BLs), 20-30% of sporadic BLs, 25-40% of immunodeficiency-associated BLs, EBV-positive DLBCL NOS and most NK/T-cell lymphomas [148][149][150][151]. In a cohort of 1253 patients with DLBCL, PD-L1 protein expression was significantly associated with EBV positivity in the non-GCB subtype and showed a trend toward inferior OS in these patients [152].
Circulating tumor DNA (ctDNA), sometimes referred to as liquid biopsy, reflects tumor DNA spread within the circulation [163]. Plasma TMB (pTMB) has been investigated in solid tumors and DLBCL as a surrogate to quantify overall tumor burden and mutational load and more accurately capture molecular tumor heterogeneity and clonal evolution [164,165]. To our knowledge, only a limited number of clinical studies have been conducted on lymphoma. One study evaluated pTMB in cHL, showing that higher baseline ctDNA and a sharper ctDNA decrease (>40%) significantly correlated with better clinical responses to sintilimab [166]. Another study, reported as an abstract only, compared TMB and pTMB in several NHL subtypes and concluded that there is a higher TMB in DLBCL than in other lymphoma subtypes [167]. Based on these results, it seems that a higher TMB seems to correlate to PD-1 ICB response, as this is the case for other solid cancers [168].

MSI and d-MMR
Microsatellite instability (MSI), caused by deficiency of the DNA mismatch repair (MMR) system, results in a higher mutational load and tumor antigen expression, leading to increased antitumor T-cell activation. MSI and MMR status are well-recognized prognostic factors in several solid tumors. The National Cancer Institute recommends a panel of five microsatellite markers for MSI tumor detection (two mononucleotide repeats: BAT-25, BAT-26; three dinucleotide repeats: D2S123, D5S346, D17S250). MSI-high tumors are defined as having instability in two or more of these markers [169]. Like those harboring a high TMB, patients with MSI-high tumors achieve durable responses to ICB [170]. Pembrolizumab and dostarlimab, both anti-PD-1 antibodies, are now approved by the FDA for the treatment of unselected advanced-stage tumors with MSI/dMMR [171]. Even though MSI-associated hypermutation represents a potential biomarker for the efficacy of PD-1 blockade, it is likely infrequent in lymphoma; reported frequencies are low, occurring in only 0.46% of cHLs, 3.2% of DLBCLs and 8% of PMBCLs [160,161,172].

MHC Expression
Antigen presentation by major histocompatibility complex (MHC) molecules is an essential step in T-cell recognition and tumor cell eradication, as T-cell activation does not occur at the tumor site but in the lymph nodes. The MHC I complex, including the beta-2-microglobulin (B2M) subunit, presents tumor-generated peptides at the cell surface, which can be recognized by cytotoxic CD8+ T cells. On the other hand, MHC II complexes present tumor antigens to CD4+ T cells [173]. Acquired mutations in the antigen processing and presentation molecules are a potential mechanism of tumor escape [174]. B2M mutations resulting in decreased MHC I expression are a well-described acquired resistance mechanism to PD-1/PD-L1 ICB in melanoma [175]. By contrast, in a study on cHL, Roemer et al. (2016) discovered that over 75% of patients had decreased or absent expression of B2M/MHCI and MHCII on tumor cells [176]. Nonetheless, patients with cHL are known to have a high response rate to PD-1 ICB. The postulated alternate mechanism triggering a response to PD-1/PD-L1 ICB could be MHC class II expression. Indeed, melanoma patients with low or absent B2M/MHCI expression but increased MHCII expression demonstrated higher responses to ICB [177]. This suggests that MHCII expression could be a potential biomarker for PD-1/PD-L1 ICB response and requires further evaluation.

Intratumoral CD8+ T Lymphocyte Infiltrate Density
Several studies have demonstrated that response to PD-1/PD-L1 ICB seems tightly related to immune cell infiltration. High levels of cytotoxic CD8+ T lymphocytes at the TME have been independently linked to improved outcomes in several lymphomas (i.e., DLBCL [178], PTL [179], FL [180], MZL [181] and HL [182,183]). The association between tumor CD8+ T lymphocyte infiltration and response to PD-1/PD-L1 ICB has been confirmed for solid tumors through a large metanalysis conducted by   [184]. Such data have not yet been reported for lymphoma.

MicroRNAs
MicroRNAs (miRNAs) are small sequences of noncoding RNAs acting as gene expression regulators. miRNAs are involved in various physiologic and pathologic processes, including immune responses [185]. Numerous miRNAs are recognized as prognostic biomarkers and are investigated as potential therapeutic targets in solid and hematologic tumors [186]. Among all miRNAs, microRNA-21 (miR-21) is one of the most frequently overexpressed miRNAs in solid tumors and B-NHL [187]. High plasma miR-21 levels have been associated with poor prognosis in several B-NHL subtypes (e.g., primary gastrointesti-nal DLBCL [188], DLBCL [189], Burkitt lymphoma [190], PCNSL [191]). miR-21 depletion may enhance antitumor activity through the polarization of macrophages into an M1-like phenotype; on the other hand, miR-21 also upregulates PD-L1 expression [192]. Taking advantage of these properties, Xi et al. recently showed in a preclinical model a synergetic effect of miR-21-depleting therapy and PD-1 ICB [192].

Gut Microbiome
Recent evidence shows that the biodiversity of the gut microbiome could influence the antitumor activity of PD-1/PD-L1 ICB; this was notably investigated in melanoma [193,194]. Namely, Liu et al. (2021) demonstrated that a favorable gut microbiome characterized by its diversity and the presence of specific bacteria species could influence the innate and adaptive immune system by increasing antigen presentation and augmenting T-cell response. On the other hand, antibiotic use may disrupt the gut microbiome and impair cytotoxic T-cell responses against tumor cells [195]. Hwang et al. studied this association in a small retrospective cohort of 62 cHL patients treated with ICB and observed that prior and/or current antibiotherapy was linked to inferior outcomes [196]. Recently, Casadei et al. (2021) prospectively collected feces (at baseline, before each treatment, at response assessment and for grade >2 adverse events) from cHL (n = 12) and PMBCL (n = 5) patients undergoing PD-1PD-L1 ICB. They reported that the results of the first six patients (all cHL) showed clear differences in their microbiomes, with a depletion of health-promoting microbial components compared with healthy controls [197].

Conclusions
PD-1/PD-L1 ICB is now a well-recognized therapy against specific lymphoproliferative disorders such as cHL and PMBCL. Besides those specific indications, PD-1/PD-L1 ICB shows generally disappointing results across all NHLs. Some patients with specific NHL subtypes, (e.g., PMBCL, PTL, PCNSL) or particular molecular findings (e.g., high TMB), may derive prolonged clinical benefit from these agents. To date, the identification of discriminative biomarkers of response to PD-1/PD-L1 ICB has been very challenging. In lymphoma, PD-L1 expression is driven by genetic alterations of the 9p24.1 locus, activation of the JAK/STAT signaling pathway and EBV infection. However, its expression is highly variable among lymphoma subtypes and does not always correlate with clinical responses to PD-1/PD-L1 ICB. Additionally, the value of a PD-L1 H-score cut-off is currently not fully understood. Other biomarkers, such as high TMB and high MSI, have not been clearly established for lymphoma, so further research in this area is urgently needed.
Author Contributions: S.P.-P. and N.L. drafted the whole manuscript. A.B. was responsible for reviewing the pathological aspects of the manuscript. S.S. and N.L. contributed to reviewing and refining the manuscript. All authors have read and agreed to the published version of the manuscript.
Funding: This research received no external funding.

Conflicts of Interest:
The authors declare no conflict of interest.