by ecancer reporter Clare Sansom

Enhancing a patient’s immune response to a tumour by blocking mechanisms that inhibit that response is considered to be a promising new type of cancer immunotherapy.

This technique, which is known as immune checkpoint blockade, has been shown to produce clinical responses in a variety of tumour types, not only those that are traditionally thought of as ‘immunogenic’.

The first registered drug in this class is ipilimumab, a fully human monoclonal antibody directed against cytotoxic T lymphocyte-associated antigen 4 (CTLA-4), a protein receptor that down-regulates the immune system.

This antibody was licensed by the FDA in 2011 following two successful Phase III trials in advanced metastatic melanoma.

However, it is only effective in about 10-15% of patients, there are no known biomarkers to predict which patients are likely to benefit, and its mechanism of action is still poorly understood.

It has been thought likely that its action is linked to regulatory T cells (Tregs) and antigen-presenting cells rather than to effector T cells

A group of researchers led by Emanuela Romano at the University Hospital of Lausanne, Switzerland has now investigated the mechanism of ipilimumab further by comparing peripheral blood mononuclear cells (PBMCs) and matched melanoma metastases from patients who respond to the antibody and those who do not.

Blood and tumour samples were extracted from 29 patients diagnosed with stage IV melanoma, 15 of whom had shown an objective partial or complete response to the drug.

The demographics of all patients were broadly similar; all patients had received at least one prior systemic therapy, and most received four cycles of ipilimumab.

Peripheral blood samples were obtained at baseline, during and after ipilimumab therapy.

Monocytes in the blood samples were divided into three subsets according to the expression pattern of HLA-DR, CD14 and CD16 using multicolour flow cytometry.

Interestingly, blood obtained from the patients who responded to ipilimumab at baseline contained higher relative and absolute numbers of non-classical, FcγRIIIA(CD16) expressing monocytes, but equivalent numbers of the other monocyte subsets.

This type of monocyte is characterised by high surface expression of HLA-DR and CD11c as well as CD16, but low surface expression of CD14, CD11b and CD163.

Mice have a type of monocyte termed FcγRIV which is orthologous to the human FcγRIIIA(CD16) monocyte and which expresses a similar pattern of receptors on its surface.

The researchers speculated that the human monocyte subset would function in a similar way to its mouse ortholog, which is known to depend on IgG and to deplete regulatory T cells (Tregs).

They also suggested that this action would depend on the antibody binding both to CTLA-4 on Tregs and to FcγRIIIA on the CD16-expressing monocytes.

They therefore obtained samples of the non-classical CD14 CD16 and of classical CD14 CD16- monocytes from the blood of healthy volunteers and co-cultured these with T cells expressing different levels of CD25 in the presence or absence of ipilimumab.

The T cells with the highest levels of CD25 expression also expressed the highest levels of CTLA-4 and of Foxp3.

In the presence of ipilumumab, the CD14 CD16 cells but not the CD14 CD16- cells were able to induce lysis in Treg cells that expressed CTLA-4 by antibody-dependent cell-mediated cytotoxicity (ADCC).

Importantly, these CD14 CD16 cells were also able to lyse similar Treg cells that had been obtained from patients with metastatic melanoma.

Romano and her co-workers then examined samples of metastatic tumour tissue obtained from ipilumumab responders and non-responders both before and after treatment for the presence of different types of macrophage and T cell using immunohistochemistry.

Tumour samples from responders and non-responders expresses similar numbers of tumour-infiltrating Foxp3 Treg cells at baseline, but after treatment the numbers of these cells had dropped considerably in the tumours from the responding patients only.

Treg cells are a known part of the immune-suppressing tumour micro-environment, so depletion of this cell type may favour an anti-tumour immune response.

Tumours taken from responding patients also had a higher density of CD16 CD68 macrophages and a lower density of CD16 CD163 macrophages than those from patients who did not respond to the antibody.

Taken together, these results suggest that ipilumumab may target and lyse immune-suppressing Treg cells via non-classical, CD16-expressing monocytes, and that cell surface expression patterns of monocytes and macrophages might be useful biomarkers of response to this antibody.

However, the researchers emphasised that this was a small study and that analysis of data from larger numbers of patients will be necessary to validate their results.

Reference

Romano, Kusio-Kobialka, Foukas et al, Ipilimumab-dependent cell-mediated cytotoxicity of regulatory T cells ex vivo by nonclassical monocytes in melanoma patients, PNAS, published online ahead of print 27 April 2015.