Abstract
Central nervous system (CNS) infections cause significant morbidity and mortality. Pathology results from tissue destruction and cerebral edema, which can lead to elevated intracranial pressure, a potentially life-threatening medical emergency. Despite decades of research, it remains unclear how the adaptive immune system senses pathogens that have invaded the brain and how cerebral edema is resolved. These processes have been challenging to understand because the brain parenchyma is devoid of lymphatic vessels, which in other tissues deliver antigen directly to lymph nodes for T cell activation and remove excess fluid that has accumulated as a result of neuroinflammation. The recently described meningeal lymphatic system has challenged our conception of how these processes are regulated in the brain. Here, a set of investigations is described that reveals how meningeal lymphatic drainage contributes to T cell immunity and fluid homeostasis in the brain in response to Toxoplasma gondii infection.
T. gondii is an intracellular protozoan parasite that infects one-third of the world’s population and can cause severe neurologic disease in immunocompromised patients. The parasite causes a similar course of infection in mice as in humans, making it a suitable model for studying how the immune system interfaces with the brain. Early studies revealed that type 1 immune responses, characterized by CD4+ and CD8+ T cell secretion of IFN-γ, are necessary for parasite restriction in the brain. Maintenance of the host-protective T cell pool in the brain requires continual recruitment of T cells from the periphery. How brain-derived microbial antigen elicits T cell responses outside the CNS is poorly understood. We provide evidence that meningeal lymphatic drainage is essential to this process. We demonstrate that surgical ligation of the collecting lymphatic vessels prevents trafficking of CD103+ dendritic cells to the deep cervical lymph nodes, reduces activation of dendritic cells that reside at this site, and leads to a reduction in activation, proliferation, and function of parasite-specific T cells. Despite this impairment, T cell responses and parasite control in the brain remained intact after disruption of meningeal lymphatic outflow. Antigen-dependent T cell proliferation in lymph nodes that do not drain CNS tissue suggest that non-CNS sources of antigen help maintain T cell responses in the brain. Thus, while meningeal lymphatic drainage supports T cell responses against T. gondii, alternative sites of T cell activation make this pathway dispensable for host protection.
Blood-brain barrier disruption frequently manifests during CNS infection and can lead to the formation of cerebral edema. We investigated whether meningeal lymphatic drainage contributes to the resolution of edema fluid in the brain. We began our studies by showing that T. gondii infection in mice leads to a multifocal pattern of vasogenic edema throughout the brain, consistent with the distribution of edema observed by MRI studies in patients with cerebral toxoplasmosis. We also discovered that infection leads to an impairment in meningeal lymphatic function, as measured by CSF tracer outflow to the deep cervical lymph nodes. We hypothesized that recovery of meningeal lymphatic function would improve resolution of cerebral edema, as previous studies have shown that edema fluid is transported from the brain parenchyma into the CSF-filled subarachnoid space. However, while delivery of a VEGF-C expression vector increased meningeal lymphatic growth and enhanced CSF outflow, there was no change in brain water content. This result reveals an important limitation in the ability of CNS-draining lymphatic vessels to regulate fluid accumulation in the brain during infection.
To conclude, this work provides novel insight into the function of the meningeal lymphatic system during CNS infection. To our surprise, we found that CNS lymphatic drainage is dispensable for maintenance of host-protective T cell responses against T. gondii in the brain and that enhancing meningeal lymphatic function does not improve control of cerebral edema. Nevertheless, meningeal lymphatic drainage was found to promote robust T cell responses against brain-derived antigen, a finding that will likely help investigators trying to strengthen T cell responses against brain tumors, and we discovered that lymphatic outflow of CSF could be restored using VEGF-C treatment, highlighting the therapeutic potential of this approach.
