There is a strong pre-clinical rationale to combine anti-VEGF therapies with ICIs (121, 122) and this combination has proven its efficacy in other cancer types, notably renal cell carcinoma (123). Angiogenesis and immune infiltrates are strongly correlated in the TME, and immune cells express angiogenic receptors.

Wilky et al. reported on a phase 2 single-arm, single centre trial of axitinib and pembrolizumab association (124). This trial included 33 patients, with an intention-to-treat (ITT) population of 32 patients. The ORR of the ITT population was 25%, the three month PFS rate was 65.6% and median PFS was 4.7 months. Notably, this trial included six LMS, two DDLPS, five UPS and 12 ASPS. Results in the ASPS population were particularly interesting with an ORR of 54.5%, a three months PFS rate of 72.7% and a median PFS of 12.4 months. In the non-ASPS population (N=21), ORR was 9.5%, the three months PFS rate was 61.9% and median PFS was 3 months.

The IMMUNOSARC trial is a phase 1b/2 trial of nivolumab and sunitinib in STS (125). The recommended phase 2 dose was set at 37.5mg daily sunitinib as induction therapy for two weeks followed by 25mg daily sunitinib in combination with nivolumab. As a whole, the ORR was 21%. The six months PFS rate was 48% and median PFS was 5.6 months.

Regarding immunotherapy combinations with chemotherapy, these are now standard-of-care in non-small-cell lung cancer (12). The hope of these combinations is that induction of immunogenic cell death by chemotherapy could synergize with immunotherapy and render it more effective (126–129). Moreover, combination of chemotherapy prevents hyperprogressive disease which has been reported in some cases of STS (130). Regional hyperthermia is a commonly used treatment in locally advanced sarcomas in combination with neoadjuvant chemotherapy (131). Even if no trial has tried to associate it with ICI to our knowledge, it should be noted that regional hyperthermia and neoadjuvant chemotherapy increase the count of TILs and reduce FoxP3+ infiltration, providing a rationale for their association with ICI (132).

The intra-tumoral approach has gained growing interest in the immunotherapy field as it yields the promise of greater benefit for less (147). Moreover, regarding immune-based cancer treatments, TAAs are the primers and the targets, and are abundantly present in the tumor (148, 149). A variety of intra-tumoral approaches exist and are promising in the sarcoma field, the most advanced is an oncolytic virus approach (150).

Mechanisms of oncolytic viruses are double: direct lysis of tumor cells and induction of an immune response by immunogenic death (151). Other mechanisms of action, notably on the TME, enhance their activity (152). Oncolytic viruses are genetically modified in order to target tumor cells, which can be done by using specific traits of tumor cells such as TERT polymerase activation, RAS signalling or TP53 pathway (153). Toxicity seems acceptable, but their efficacy can be limited by multiple factors: clearance of virus by the immune system or induction of exhaustion on lymphocytes of the TME, for instance.

In line with these limitations, combination therapy with ICIs hold great promise to synergize with oncolytic viruses and potentiate their activity (154–159).

Kelly et al. recently reported an impressive ORR of 35% in 20 patients with STS in the advanced setting treated by the combination of Talimogene laherparepvec (T-VEC) and pembrolizumab (160). T-VEC is a herpes simplex virus type 1 oncolytic virus, injected intratumorally and approved by the US Food and Drug Administration (FDA) for treatment of melanoma (161). The combination of T-VEC intratumorally and pembrolizumab intravenously at 200mg flat dose was administered every 21 days. PFS rate at 12 weeks was 70%, PFS rate at 24 weeks was 39.4% and median PFS was 17.1 weeks.

Vaccination against cancer includes a variety of modalities and targets. The general idea is to stimulate the adaptive immune system against TAAs presented to lymphocytes by antigen-presenting cells. The antigens are tumor specific and can either be identified targets with specific cancer neoantigens or whole tumor lysates.

The vaccination method can be immune cell-mediated, using antigen presenting cells such as dendritic cells, peptide based or nucleic acid based (162, 163). As a whole, these methods have shown exciting pre-clinical data and interesting early phase trials with limited activity in monotherapy.

It is likely that these immunotherapies will be further developed in combination with other treatment modalities: chemotherapy, radiotherapy or ICIs, in a prime and boost approach. Most of the clinical research in the STS field has been focused on CTAs.

Leiomyosarcoma accounts for roughly 10-20% of all STS, and has a crude incidence rate of around 0.7/100.000/y (178). Two distinct subtypes are usually distinguished: uterine LMS (ULMS) are less frequent than soft-tissue LMS (STLMS). The TCGA cohort helped define the molecular biology of these tumors: LMS is globally associated with complex genomics and a high number of CNA (52). On the other side, the mutational burden is low, with a median mutational burden ranging from 1.5 to 2.5mut/Mb, and only 0.7% (STLMS) and 0.9%(ULMS) harbor more than 20mut/Mb (51, 64). The most frequent copy number loss affects chromosome 10 (loss of PTEN) (179), and on the whole, molecular alterations often concern TP53, RB1 as well as PTEN. The PIK3CA/AKT pathway is often hyperactivated (180), without a clear difference between STLMS and ULMS. The two subtypes however have distinct methylation profiles and mRNA signatures: ULMS have a higher DNA damage response score, are characterized with homologous recombination deficiency (HRD) in 20% of cases, and with a loss of BRCA2 in 6.5% (181). Moreover, ULMS is known to express hormone receptors in around 40% of cases (182). On the other side, STLMS have a more activated hypoxia-induced factor-1α (HIF-1α) pathway.

On the whole, the sensitivity of LMS towards chemotherapy is moderate, and first-line treatment relies on doxorubicin-based regimen (31) or on gemcitabine-docetaxel association (183). In later setting, options rely on chemotherapy with dacarbazine, trabectedin, eribulin. Antiangiogenic therapies, especially pazopanib, have also demonstrated a meaningful activity in LMS (184). Despite these treatment options, metastatic LMS still has a poor prognosis with an overall survival around 14 months (185).

As we previously described, LMS is characterized by complex genomics with a high number of CNA, but a lower mutational burden than UPS (64). On the whole, LMS is to be considered as poorly infiltrated by CD8+ T cells (88, 186), even though there is an heterogeneity, with a richly infiltrated subset. TAMs are more represented, although less than in other sarcomas of complex genomics, and M2/M1 ratio is high and correlated to a homologous recombination deficiency, CNAs but also to a poorer prognosis (61, 187). In studies using RNA-sequencing, LMS express PD-L1 (62, 63), LAG-3 and TIM-3 in around 50-60%, 15% and 10% (64) of cases, respectively.

Petitprez et al. analysed 189 LMS in their cohort: most of them were classified in the SIC A (immune desert) and SIC B (immune-low), and only a few tumors harbored TLS, which were described as rather immature and localized in the periphery of the tumor (84).

Results of phase II trials have been quite disappointing in advanced LMS so far, with an ORR of 6.9% and a non-progressive rate of 54% (47). These results are corroborated by a phase 2 trial showing no response to anti-PD1 monotherapy (43), and by a better comprehension of the TME of LMS. New approaches are therefore needed to improve the outcome of this disease.

As we described, PTEN is lost in around 60% of LMS (179), and PIK3CA/AKT pathway is therefore often pathologically activated, leading to cell proliferation and to an inhibition of apoptosis. This loss of PTEN was also identified as a resistance mechanism to anti-PD1 therapy in a metastatic LMS (188), which was also found in other tumor types (189). Therefore, it would be interesting to associate PD-1 blockade to PIK3CA/AKT inhibition (PIK3CA or mTOR inhibitors).

It has also been found that IFNγ-induced JAK-STAT pathway activation was responsible for an increase in PD-L1 and IDO-1 expression in metastatic LMS, providing the rationale for an association of PD-1 and IDO blockade (190).

One of the issues in LMS is its poorer CD8+ infiltrate, compared to other complex genomics sarcomas, probably hampering its response to ICI. It would be interesting to increase its antigenicity and radiotherapy could be an interesting option (191). Associating ICI with PARP inhibitors to induce more neoantigens could also be an interesting attitude, especially in ULMS, associated with HRD deficiency. A last strategy would be to associate ICI with chemotherapy: clinical trials associating anti-PD1 to gemcitabine or eribuline are ongoing (see Table 2 ).

Liposarcomas have an incidence around 0.8-1/100.000/year. This group of sarcomas is composed of three distinct entities: well-differentiated and dedifferentiated LPS (WDLPS and DDLPS), MRLPS and pleomorphic LPS (192, 193).

WDLPS and DDLPS represent 50% of LPS, the primitive tumor localization is retroperitoneal in 45% of cases and they are characterized molecularly by the supernumerary ring chromosome and/or giant marker chromosome of amplified 12q13-15 region, leading to the amplification of MDM2, CDK4 and HMGA2 genes (194). These LPS are less chemosensitive than other LPS (195), although combination chemotherapy with anthracycline remains the most active regimen (196, 197). WDLPS have a more indolent course, tend to recur locally and in 10% of cases have a DDLPS component.

MRLPS account for 40% of LPS, they are characterized in >95% of cases by a chromosomal translocation involving chromosomes 12 and 16, leading to the fusion protein FUS-DDIT3. Importantly, 95% of MRLPS express NY-ESO-1 CTA (71) in immunohistochemistry. Myxoid LPS is the more indolent counterpart of round cell LPS, which are defined by having more than 5% of small round cells in a myxoid LPS. MRLPS are radiosensitive (198) and more chemosensitive than DDLPS/WDLPS (195), with a particular responsiveness regarding trabectedin (199).

Pleomorphic LPS account for 5% of LPS, are more aggressive and poorly characterized.

As a whole, LPS seem to be infiltrated by T-cells and express PDL1 (63, 186). In regards to separate histotypes of LPS, a general pattern seems to emerge: DDLPS are more infiltrated and express more PDL1, whereas MRLPS seem to be less infiltrated, and WDLPS are probably somewhere in the middle (62, 100, 107, 200), though the latter has frequently been grouped with DDLPS in reported studies.

DDLPS and WDLPS seem to have an expression of roughly 10-20% PDL1 (62, 94, 108, 200, 201), both in immunohistochemistry and gene expression studies. Compared to other STS, DDLPS seem to be in the higher proportions of TILs and PDL1 expression, though consistently reporting less infiltration than certain subtypes such as UPS (62, 100, 108, 202). TILs are present in over 50% of cases (61, 108, 200) with presence of NK cells, B-cells, TLS and expression of co-inhibitory checkpoints such as LAG3 and TIM3 (100, 201, 203). However, T-cell infiltration seems to be less oligoclonal than LMS or UPS (62). Regarding TAMs, DDLPS are amongst the STS containing the most TAMs, but with an important infiltration by T-cells so the ratio TAM/TILs is in the average, and close to the WDLPS ratio (61).

MRLPS are poorly infiltrated by T-cells, have a low PDL1, low TCR clonality and are infiltrated by TAMs with a majority of M2 type macrophages (62, 100). PDL1 expression is usually less than 10% (94, 200). The importance of macrophages and their prognostic value has been reported specifically in MRLPS (204). Interestingly, 18.5% of MRLPS are mutated for the PI3KCA gene which correlates with the absence of TILs (200) and the presence macrophages (204). These tumors are amongst those with the highest ratio of M2/M1 TAMs and expressing the most SIRPα (61).

Regarding pleomorphic LPS, data is scarce and inconsistent. As a whole, these tumors have markers of bad prognosis, with little to no T-cell infiltration (94, 200), absence of TLS (201) and high TAMs and M2 infiltration (61).

As discussed previously, results of ICI in LPS have been promising at first, with strong rationale as these tumors are infiltrated and express PDL1. However, further assessment has been disappointing with an ORR of roughly 8% (47).

DDLPS and WDLPS seem to have some degree of response to ICIs, but monotherapy is insufficient. Interesting results have been reported in combination with chemotherapy, as displayed in Figure 1 (136, 137). Another perspective could be combinations with molecularly targeted therapies, namely MDM2 antagonists and anti-CDK4/6. MDM2 antagonists have achieved interesting results as monotherapy in DDLPS, though with important myelotoxicity (205). Importantly, MDM2 had been suggested as a mechanism of hyperprogression in ICIs (206), making it a good target for combination with ICIs. Moreover, interesting preclinical data shows antagonizing MDM2 enhances T-cell killing (207) and NK-cell mediated killing (208) with a synergism when combined to antiPD1 (209). Likewise, CDK4/6 inhibitors have shown limited responses in LPS (210) but trigger antitumor immunity (211), which could potentially synergize with ICIs. Epigenetic modifications are crucial in LPS (212) and synergism between epigenetic drugs and immunotherapy has a strong rationale (213). Finally, combination with other immune-based therapies could be efficient, notably other ICIs, as DDLPS are the STS type with the highest expression of LAG3 and TIM3 (100).

As discussed previously, MRLPS have high expression of NY-ESO-1 CTA, low TILs and low clonality of TCR. Results of modified TCR T-cell based immunotherapy have been promising, and this seems to be an efficient therapy, which is being tested in earlier phases of the disease. Another interesting option to pursue, might be combination with trabectedin, as this drug is immunogenic (138, 214) and particularly efficient in MRLPS (199). Likewise, radiotherapy combinations with ICIs hold great promise (215): radiotherapy increases PDL1 expression in STS (93) and MRLPS are particularly sensitive to radiotherapy (198). Moreover, combination of radiotherapy and trabectedin seems interesting in MRLPS (216), and might be combined as a whole with ICIs.

Regarding pleomorphic LPS, more translational and clinical data for this specific histotype is needed in order to better define its sensitivity to immunotherapy.

Undifferentiated pleomorphic sarcomas used to be called malignant fibrous histiocytoma (MFH). These sarcomas are characterized by their absence of distinct features of differentiation (217–219).

UPS/MFH was the most frequent type of STS but as molecular phenotyping techniques have progressed, it seems that misdiagnosis was a frequent problem and differentiation of certain phenotypes was identified within certain misclassified UPS/MFH (220–223). However, UPS remains one of the three most frequent histotypes of STS with LPS and LMS (52) and one of the worst prognosis (224).

UPS are complex genomic sarcomas, have the highest CNA and mutation rate with frequent alterations of TP53 and RB1 (52, 221). Multiple studies have tried to better characterize their molecular background (225–231). Of note, some studies have suggested that there is a continuum between MFS and UPS (52) whereas others have found LMS and UPS to be molecularly similar (232–235). Investigations report on different molecular alterations with different potential prognostic (230–232, 234, 236–238) or therapeutic implication (221). Other studies have shed light on subsets of UPS with alterations on the PI3K/Akt pathway (225, 237), the Hippo pathway (52, 234) or RAS/MAPK pathway (225, 230, 237) with particular attention to FGF (226, 227). In particular, recently a multi-omics approach has separated UPS in two molecular subtypes: one immune-high and one expressing FGFR (227), which is consistent with previous data (229). As a whole, UPS is probably a heterogeneous histotype, with distinct molecular subtypes which might benefit from more extensive molecular testing before treatment (220).

Looking at tumor genome, UPS have amongst the highest TMB in sarcomas, with angiosarcomas (51, 225). In line with this higher mutation rate, T-cell infiltrates in UPS have a more oligoclonal TCR repertoire than other sarcomas (62).

As a whole, UPS are infiltrated by TILs (64), with notably presence of CD8+ and FoxP3+ T-cells (91, 100, 239) and TLS. TILs express immune checkpoint proteins, though less than in DDLPS, with 36% of cases with PD1+ TILs, 72% of cases with LAG3+ TILs and 83% TIM3+ TILs (100) by immunohistochemistry. PDL1 is highly expressed, roughly 30-50% of cases (94, 106).

TAMs are predominant, with a clear majority of M2 phenotype. In fact, UPS are the second histotype most infiltrated by TAMs, after angiosarcomas, and share the highest M2 proportion with LMS. Notably, UPS have a low expression of CD47 and moderate SIRPa expression on TAMs (61).

This histotype microenvironment seems to be more immune infiltrated than other STS (62). However, as previously mentioned, this seems to be particularly true for a subset of UPS (227).

Up to now, clinical data of ICI in UPS have reported ORRs of 15-40% with ICI alone and over 50% when combining ICIs with other molecules (47), amongst the highest. For UPS, the next step is probably better selection of patients with biomarkers for personalized medicine. In the immune-high subset of UPS, ICIs might be particularly boosted by combination with radiotherapy, which has proven to increase PDL1 expression in STS (93), and particularly UPS (36, 240).

Notably, UPS are particularly responsive to combination of ICIs (41). For other molecular subtypes, driven by particular targets, combination ICI with FGFR inhibitors might be beneficial (241) and should be further explored. Combination of ICI with oncolytic virus is also of great interest in this histotype: in the phase 2 trial evaluating Pembrolizumab with T-VEC, the 2 patients with UPS treated with this regimen had a partial response.

Translocation-related sarcomas are driven by a chromosomal translocation, leading to the expression of a fusion protein. For clarity reason, we chose to focus only on synovial sarcomas (the most frequent subtype) and ASPS (with an impressive response rate to ICI).

Synovial sarcoma accounts for nearly 10% of all STS and can occur in any anatomic site, preferentially in adults younger than 30 years old (242). It is characterized by the presence of a pathognomonic translocation between chromosome X and chromosome 18, t(X,18)(p11.2;q11.2) leading to the expression of an SS18:SSX fusion protein. This SS18:SSX protein binds and to the BAF (BRG1-associated factors) complex and activates it, thus displacing the tumor suppressor INI1 (coded by the gene SMARCB1). This epigenetic phenomenon is eventually responsible for a repression of E-cadherin, a downregulation of BCl2, and a downregulation of MCL1 (243–245). Another consequence is the pathological expression of CTAs, such as NY-ESO-1 (see supra). These epigenetic mechanisms in synovial sarcoma can be emphasized by an overexpression of EZH2 (enhancer of zeste homologue 2), which is described in poorly differentiated synovial sarcomas, and seems associated with a poorer prognosis (246).

Despite a relatively good comprehension of its biology, the prognosis of synovial sarcoma remains poor, although chemosensitive. In the metastatic setting, treatment relies on anthracycline-based chemotherapy, but another option is high-dose ifosfamide alone, to which synovial sarcoma is known to be sensitive (247). Pazopanib and antiangiogenic therapies are also particularly interesting in this particular histotype (184, 248).

ASPS is a much rarer sarcoma subtype, accounting for less than 1% of all STS and occurring preferentially in young adults as well (249). Even though around 55% are metastatic at diagnosis, they tend to have a more indolent course than other histological subtypes, with a 5-year overall survival of 61% in case of a metastatic disease (250). It is characterized by the t(X;17)(p11;q25) translocation, which codes a chimeric ASPSCR1-TFE3 transcription factor, and is indirectly responsible for an overexpression of genes related to angiogenesis (among which c-Met or VEGF), cell proliferation and metastasis (251). ASPS is known for its sensitivity to TKI therapy with a response rate around 30% (252, 253), counterbalancing a poor response rate to anthracycline chemotherapy (254).

Translocation-related sarcomas are supposedly less immunogenic, due to a poor immunogenicity of fusion proteins and a lower mutational burden. Synovial sarcoma is no exception to this with a median TMB around 1.7mut/Mb (51) and only 1% of tumors harboring more than 20mut/Mb. However, they are characterized by a high CTA expression: as we previously described, 49-76% of synovial sarcomas express NY-ESO-1 and 51% of them coexpress NY-ESO-1 and MAGE-A4 and PRAME (72–74).

Translocation-related sarcomas have a poorer CD3+ infiltrate than non translocation-related sarcomas and 21% of them show no TIL (100). Concerning synovial sarcomas, this immune desert is also associated with a low TCR clonality and a lower expression of antigen presentation genes (62). They also have one of the lowest TAM infiltrate (61) across all STS subtypes. They are associated with a resting mast cell and a naive B cell signature. Moreover, immune-checkpoint proteins are rarely expressed in translocation-associated sarcomas: 64% are described as PD-1, LAG-3 and TIM-3 negative in immunohistochemistry (100). In synovial sarcoma, there is no PD-L1 expression by tumor cells, and only 20% of them have PD-L1 expressing TAMs in immunohistochemistry (88). In Petitprez et al., SIC E synovial sarcomas were a minority (20%), whereas SIC A and B (immune deserts) represented around 50% (84).

Data regarding TME of ASPS is scarce due to the tumor rarity. They are described as poorly infiltrated as well, with a low CD3+, CD8+ and FoxP3+ infiltrate (100). Among translocation-associated sarcomas, they are however described as having the highest CD163+ expression, responsible for one of the highest M2/M1+M0 ratios (61).

Given the overexpression of CTA, synovial sarcoma was initially considered as a good candidate for ICI. The first trial used anti-CTLA-4 monotherapy with ipilimumab in 6 patients, but no response was observed (42). Overall, ICIs have failed to demonstrate a meaningful clinical benefit in synovial sarcoma, with a response rate under 10%, if no response at all (see Table 1 and Figure 1 ). This is probably the result of the global immune desert of synovial sarcomas. However, if epigenetic mechanisms play a major role in the oncogenesis of synovial sarcomas, the synergy of epigenetic drugs with ICI should be assessed in this tumor type, for example with an association of EZH2 inhibitors and ICI in tumors overexpressing EZH2.

With the association of specific neoantigens (NY-ESO-1 and MAGE-A4) and the absence of immunosuppressive microenvironment, the future of immunotherapy in synovial sarcoma probably lies in the targeting of specific neoantigens. Adoptive cellular therapies targeting NY-ESO-1 (171) have shown impressive results, so did engineered autologous T-cells targeting MAGE-A4 (175). Vaccine therapies, such as LV305 have also shown interesting results (165) and are currently assessed in several clinical trials (see Table 2 ). It should be noted that ICI could be of interest in combination with these therapies, as suggested by the stunning response rate of ICI after failure of CDX1401 treatment (164).

On the other hand, ASPS is probably the histotype with the most impressive benefit from ICI with a global ORR of 48.8% for anti-PD(L)1 monotherapy (47). The reason for this sensitivity remains unclear in a histotype characterized by a poor immune infiltrate.

Given its known sensitivity to TKI such as pazopanib and the known synergy between these treatments, the association of ICI and antiangiogenics needs further assessment, but results are impressing so far with the association of Axitinib and Pembrolizumab harboring an ORR of 54.5%, a three months PFS rate of 72.7% and median PFS was 12.4 months (124).