Myeloid cells in type 2 immunity: unravelling susceptibility and resistance to tissue nematode infection

Filarial nematodes (roundworms) are vector-transmitted parasites that cause substantial human and animal suffering. Litomosoides sigmodontis, is a filarial nematode in which the full life cycle can be sustained in laboratory mice, allowing us to study the immune response to these parasites. Like most parasitic worms, L. sigmodontis induces a specific arm of the immune system known as "type 2" immunity. This host immune response is important for killing these large multi-cellular parasites, but type 2 immunity is also important for tissue repair and when poorly controlled is responsible for causing scar tissue (fibrosis) that is associated with many serious non-infectious diseases.

A number of white blood cells are involved in the type 2 immune response. These include Th2 cells, which secrete molecules to direct the type 2 immune response. We are particularly interested in macrophages, a cell which becomes activated and divides when exposed to the molecules released by Th2 cells. Adult L. sigmondontis parasites lives in the pleural cavity, a fluid filled space that surrounds the lung. Following infection, the numbers of Th2 cells and macrophages expand in number in the pleural cavity. Macrophages can either live in the tissue (called resident macrophages) or can be newly recruited from the blood. We find that macrophages are very different between two genetic strains of mice. One mouse strain kills the worms in the pleural cavity but in the other strain, the worms grow and produce offspring. In the strain that kills its worms, the resident macrophages expand enormously in number, while in the strain that lets the worms survive, there is recruitment of new cells from the blood and very few resident macrophages. We hypothesize that these differences in macrophages, underlie why one strain can efficiently kill the parasite while the other cannot. Whether an immune response causes resident macrophages to expand or new macrophages to be recruited, is likely to determine the outcome of infection or injury in many different conditions. Therefore, investigating why these two strains of mice have such different patterns for macrophage expansion, will not only help explain how filarial infections are controlled, but will also be relevant to other diseases.

In the strain of mouse that is resistant to infection, Th2 cells are necessary to kill the worms, and the resident macrophages respond to Th2 signals to become activated and divide. However, we do not know the answer to a very fundamental question: Do the macrophages kill the worm? Also, typically T cells make direct contact with macrophages to activate them, but those rules do not seem to apply during this worm infection, so how do Th2 cells recognise macrophages and tell them to become activated? By manipulating the different cells in the pleural cavity during infection, we hope to answer these questions, and provide fundamental new information about type 2 immunity, in addition to revealing mechanisms of worm killing.

We have also found that infection in the pleural cavity causes fibrosis along the edge of the lung wall and it is thought that some serious lung diseases may start in the pleural cavity. Our experiments manipulating the different cell populations in the pleural cavity therefore may also provide new insights into fibrotic lung diseases. Scar tissue formation and inflammation in the pleural cavity is a very common problem among patients with many different kinds of lung disease, including Covid19, and yet the immune response in the pleural cavity is very rarely investigated. We believe that studying the pleural cavity during L. sigmodontis infection provides a unique opportunity to learn about this understudied but clinically important tissue.

Helminth infection is characterised by a type 2 immune response with polarisation of macrophages toward an M(IL-4) phenotype. It is increasingly recognised that macrophage functionality relies not only on direct cytokine signals but cellular origin and tissue environment. This programme is built around the pleural cavity, a tissue containing two predominant macrophage populations: monocyte-derived small cavity macrophages (SCMac) and large cavity macrophages (LCMac) which have a 'residency' gene expression module, conferred by the tissue niche. We will use the filarial nematode Litmosoides sigmodontis (Lito) as a model of pleural infection and as a tool to study M(IL-4) cells more generally. Mirroring divergent outcomes in human filariasis, susceptibility to Lito infection is dependent on host genotype, with striking differences in M(IL-4) cells between resistant and susceptible strains of mice. In resistant C57BL/6 mice LCMac expand greatly through proliferation and become M(IL-4) polarised. In susceptible BALB/c mice there are relatively more SCMac-like cells which appear incapable of acquiring a full LCMac-residency or M(IL-4) programme. Aim 1 seeks to explain if strain differences are a result of intrinsic differences in macrophage programming, alterations of the tissue niche or due to differences in Th2 cell activity. The next 2 aims focus on resistant C57BL/6 mice as a model of type 2 immunity culminating in worm killing. Aim 2 looks upstream of macrophages to identify the cell-cell interactions in the pleural cavity and essential co-factors that drive M(IL-4) polarisation and proliferation. Aim 3 will look downstream of M(IL-4) cells to assess their contribution to worm killing and tissue integrity in the pleural cavity using mice which lack specific residency, M(IL-4) or proliferation gene modules. This programme will advance our knowledge of the activation and function of M(IL-4) cells and provide novel information on the function of the pleural space.