Neutrophils as modulators of the immune tumor microenvironment

Tumor-associated neutrophils (TANs) make up a significant portion of the immune cell infiltrate in many cancers, and the exact mechanisms by which these cells affect tumor progression are gradually discovered. It is now well accepted that neutrophils also play a key role in multiple aspects of cancer biology. We have studied the many different ways by which TANs impact other tumor-infiltrating immune cells. TANs actively secrete cytokines and chemokines, modifying the recruitment and polarization of various immune cells in the tumor microenvironment, actuating as regulators of the immune system.

In several studies we have done, we isolated TANs from lung adenocarcinoma primary tumors, and evaluated their capacity to recruit, activate or inhibit immune cells in vitro, i.e. on cells isolated from spleens of tumor-bearing mice. We further assessed the in vivo effects of TANs, e.g. by using neutralizing antibodies to TANs or to specific chemokines/cytokines.

In previous work we demonstrated the active secretion of CCL17 by TANs followed by active recruitment of CD4+ T-regulatory cells (T-regs) into the tumor microenvironment. Depletion of tumor neutrophils strongly reduced the chemoattraction of T-regs. Recruiting T-regs to the tumor site induces immunologic self-tolerance and impaires immune response to tumor cells. In another study TANs isolated from murine tumors promote immunosuppression by strongly inducing CD8 T-cell apoptosis. The TNFα pathway in TANs is critical for the induction of apoptosis, involving the production of NO, but not ROS. Interestingly, TANs were found to have contradictive effects on CD8 T-cells, capable of activating these cells, but specifically induce apoptosis only of non-activated CD8+CD69- cells.

In our recent unpublished work, TANs isolated from primary tumors in LLC tumor-bearing mice, were found to attract significant amounts of monocytes (CD11b+Ly6C+), dendritic cells (CD11b+CD11c+) and B cells (CD45+C19+B220+). Neutrophil depletion significantly reduces the amount of tumor-infiltrating B cells. Our data suggest that TNFα, but not CXCL12, CXCL13 or CXCL9 play a major role in B cell recruitment by TANs. Accordingly, TANs isolated from TNFα-KO mice attracted significantly less B cells in vitro. We further identified subsets of B cells infiltrating the tumors and attracted specifically by TANs. We found tumor-infiltrating B cells to be mainly composed of B220+CD1d+CD23- and B220+CD1ddimCD23+. Isolated TANs attracted splenic B220+CD1d+CD23- cells but also appear to have the capability to activate splenic B cells, increasing significantly their expression of CD1d. Deeper characterization of these B cells subsets is currently being completed. Mounting evidences have recently documented a role for B cells in modulating the immune response to tumors. Our data suggests an active role of TANs in the differential recruitment and activation of B-cells in the tumor microenvironment.

Our studies elucidate the chemotactic forces played by neutrophils affecting immune-cells infiltration into the tumor and their activation, promoting or inhibiting the establishment of a permissive tumor microenvironment. These important mechanisms provide us with a deeper understanding of the ways these cells support or fight cancer, ultimately helping develop new strategies to direct the immune system against the tumor.