Fc: Fc gamma receptor interactions in anti-PD-1 monoclonal antibody therapy and their role in anti-tumour immunity against neuroblastoma

Abstract

Programmed cell death (PD)-1 is an inhibitory co-receptor expressed on activated T cells that plays an important role in down-regulating anti-tumour immunity. Monoclonal antibodies (mAbs) that block PD-1 have shown encouraging results in advanced adult cancers, but objective responses are limited to a subset of tumours and patients. Recent studies highlight the importance of the Fc domain of anti-PD-1 mAbs with their anti-tumour activity. Therefore, the aim of this work was to study the role of Fc: Fc gamma receptor (FcγR) interaction in anti-PD-1 therapy, together with the potential application of these mAbs in neuroblastoma, a childhood cancer that bears a particularly poor prognosis.

In vitro characterisation of three anti-PD-1 isotypes (mouse IgG1, IgG2a and IgG1-N297A) was performed to confirm binding and functional activity of these mAbs. To investigate the effect of Fc: FcγR interactions in vivo, antigen-specific T-cell responses were studied with the OT-I transfer model and endogenous responses to the model antigen ovalbumin. Anti-tumour activity and modulation of immune populations in the tumour microenvironment (TME) was assessed in an immunogenic model (MC38) as well as in murine neuroblastoma models.

Results showed that an anti-PD-1 mAb that lacked FcγR binding (IgG1- N297A) was optimal at expanding anti-CD40 stimulated antigen-specific responses of both transgenic OT-I and endogenous T cells. In the context of cancer, both IgG1 and IgG1-N297A mAbs improved survival and increased immune infiltration in MC38, although the IgG1-N297A mAb induced a more profound activation of immune cells within the TME. In contrast, preferential engagement of activating FcγRs by anti-PD-1 IgG2a caused phagocytosis of activated T cells in vitro and abrogated therapeutic activity in vivo. A similar trend in therapeutic efficacy and phenotypic changes across anti-PD-1 isotypes was observed in neuroblastoma tumours. In view of the limited efficacy as a monotherapy in this model, anti-PD-1 mAbs were combined with tumour-targeting anti-GD2 mAbs and immune-modulatory cyclophosphamide with the aim to increase T-cell infiltration and the CD8: Treg ratio. Despite improved outcomes, the triple combination schedule was not sufficient to stop tumour growth, and hence other schedules or therapeutic strategies may be required.

Taken together, these results support an important role for Fc:FcγR interactions in the therapeutic activity of anti-PD-1 mAbs, highlighting the detrimental effect of engaging of activating FcγR. Future work should focus on defining the role of the inhibitory FcγR in modulating anti-PD-1 mAb therapy. Furthermore, anti-PD-1 mAbs were also able to modulate the neuroblastoma TME, albeit to a lesser degree. Due to the cold nature of these tumours, combinatorial approaches that increase immune infiltration and activation will likely be required to enhance the efficacy of these mAbs.

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Identifiers

ORCID for Stephen Beers: orcid.org/0000-0002-3765-3342

ORCID for Juliet Gray: orcid.org/0000-0002-5652-4722

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