Resistance Mechanisms and Barriers to Successful Immunotherapy for Treating Glioblastoma.

Glioblastoma (GBM) is inevitably refractory to surgery and chemoradiation. The hope for immunotherapy has yet to be realised in the treatment of GBM. Immune checkpoint blockade antibodies, particularly those targeting the Programme death 1 (PD-1)/PD-1 ligand (PD-L1) pathway, have improved the prognosis in a range of cancers. However, its use in combination with chemoradiation or as monotherapy has proved unsuccessful in treating GBM. This review focuses on our current knowledge of barriers to immunotherapy success in treating GBM, such as diminished pre-existing anti-tumour immunity represented by low levels of PD-L1 expression, low tumour mutational burden and a severely exhausted T-cell tumour infiltrate. Likewise, systemic T-cell immunosuppression is seen driven by tumoural factors and corticosteroid use. Furthermore, unique anatomical differences with primary intracranial tumours such as the blood-brain barrier, the type of antigen-presenting cells and lymphatic drainage contribute to differences in treatment success compared to extracranial tumours. There are, however, shared characteristics with those known in other tumours such as the immunosuppressive tumour microenvironment. We conclude with a summary of ongoing and future immune combination strategies in GBM, which are representative of the next wave in immuno-oncology therapeutics.

Impaired circulating myeloid CD1c+ dendritic cell function in human glioblastoma is restored by p38 inhibition - implications for the next generation of DC vaccines.

Current treatments for glioblastoma (GBM) have limited efficacy and significant morbidity and therefore new strategies are urgently needed. Dendritic cells have the power to create anti-tumor immune responses. The greater potency of circulating dendritic cells (DC) over laboratory-generated monocyte-derived DC makes them exciting new immunotherapeutic candidates. To determine the immune status of GBM patients we initially investigated the frequency and function of circulating DC subsets. Furthermore, we tested the therapeutic potential of inhibiting the p38 mitogen-activated protein kinase pathway (p38i) in circulating DC to overcome DC dysfunction. GBM patients (n = 16) had significantly reduced numbers of the major myeloid circulating dendritic cell (cDC2) and plasmacytoid DC vs healthy controls; 1736 vs 4975 (p = 0.028) and 893 vs 2287 cells/mL (P = <0.001) respectively. This inversely correlated with dexamethasone (Dex) dose in a log-linear model, and disease status. Patients' cDC2 were immature with impaired interleukin (IL)-12 secretion, reduced IL-12:IL-10 ratio, and low HLA-DR and CD86 expression. Exposure of healthy donor cDC2 to Dex or GBM cell lysate resulted in a similar low IL-12:IL-10 ratio. Inhibition of p38 restored the IL-12:IL-10 balance in Dex or tumor lysate-conditioned healthy cDC2 and enhanced T-cell proliferation and interferon-gamma (IFNγ) production. Importantly, patient-derived cDC2 showed a similar reversal of DC dysfunction with p38i. This study demonstrates the therapeutic potential of developing the next generation of DC vaccines using enhanced p38i-conditioned cDC2. We will therefore shortly embark on a clinical trial of adoptively transferred, p38 MAPK-inhibited cDC2 in adults with GBM.

Tumour-mediated disruption of dendritic cell function: inhibiting the MEK1/2-p44/42 axis restores IL-12 production and Th1-generation.

Dysfunctional dendritic cells (DC) are common in cancer patients; however, the underlying molecular targets are poorly understood. Nevertheless, adoptive-transfer and in situ vaccination protocols continue, largely without addressing immune-suppression. Understanding tumour-mediated DC suppression would assist rational design of next generation immunotherapy. This study used a tumour-lysate loaded DC model of adoptive immunotherapy and also necrotic cells common in many tumours. Patient-derived and healthy-donor monocyte-derived DC were examined for disruptions in mitogen-activated protein kinase (MAPK) signalling pathways associated with their capacity to generate Type-1 helper-T cell populations (Th1). Melanoma-lysate markedly suppressed TLR4-dependent IL-12p40 and p70 production. Suppression of IL-12p70 occurred independently of maturation markers (e.g., CD40, CD80, CD83) and correlated with depressed p35 and p40 transcription. Decreased IL-12 secretion was not associated with IL-10, TGFbeta, VEGF, PGE(2) or ganglioside in tumour lysates or attributable to endogenous PGE(2) release from DC. In contrast to HUVEC lysate, melanoma-lysate-treated DC were less able to generate Th1-responses from naïve T-cells. The p44/42 MAPK was hyper-activated by melanoma lysate, but not that of HUVEC. Blockade of MEK1/2, the upstream kinase for p44/42, with U0126 prevented p44/42 activation, restored IL-12p70, and permitted effective generation of Th1-responses. Therefore the p44/42 MAPK is a target for tumour-mediated immune suppression of DC resulting in transcriptional down-regulation of IL-12 p35 and p40 genes, reduced IL-12 secretion and suppressed Th1-responses. Pharmacological intervention in the MEK-p44/42 axis may be applicable to render DC resistant to the suppressive effects of tumour lysates and may form part of a combination immunotherapy.