Chemokine receptors CCR2 and CX3CR1 regulate viral encephalitis-induced hippocampal damage but not seizures Christopher Käufera,1, Chintan Chhatbarb,1, Sonja Bröera,1,2, Inken Waltla, Luca Ghitab, Ingo Gerhauserc, Ulrich Kalinkeb,d, and Wolfgang Löschera,d,3 aDepartment of Pharmacology, Toxicology, and Pharmacy, University of Veterinary Medicine Hannover, 30559 Hannover, Germany; bInstitute for Experimental Infection Research, TWINCORE, Centre for Experimental and Clinical Infection Research, a joint venture between the Helmholtz Centre for Infection Research, 38124 Braunschweig, Germany, and the Hannover Medical School, 30625 Hannover, Germany; cDepartment of Pathology, University of Veterinary Medicine Hannover, 30559 Hannover, Germany; and dCenter for Systems Neuroscience, 30559 Hannover, Germany Edited by Lawrence Steinman, Stanford University School of Medicine, Stanford, CA, and approved August 7, 2018 (received for review April 20, 2018) Viral encephalitis is a major risk factor for the development of expression of the chemokine receptors CCR2andCX3CR1 seizures, epilepsy, and hippocampal damage with associated cogni- in mice, so-called “inflammatory” (or “classic”) monocytes + tive impairment, markedly reducing quality of life in survivors. The (LY6ChighCCR2 CX3CR1low), which are highly mobile and rap- mechanisms underlying seizures and hippocampal neurodegenera- idly recruited to inflamed tissues, can be distinguished from pa- − tion developing during and after viral encephalitis are only in- trolling (nonclassic) monocytes (LY6ClowCCR2 CX3CR1high), completely understood, hampering the development of preventive which are larger in size and patrol along vascular endothelium such treatments. Recent findings suggest that brain invasion of blood- as the BBB (4, 6). Brain injury is typically associated with invasion born monocytes may be critically involved in both seizures and brain of monocytes and other immune cells and concomitant amplifi- damage in response to encephalitis, whereas the relative role of cation of local inflammation (4). Brain-resident cells produce ’ microglia, the brain s resident immune cells, in these processes is not the myelo-attractant cytokine CCL2 (also known as “monocyte clear. CCR2 and CX3CR1 are two chemokine receptors that regulate chemoattractant protein 1”), a CCR2 ligand that promotes the responses of myeloid cells, such as monocytes and microglia, + INFLAMMATION transmigration of CCR2 monocytes across the BBB via CCL2/ IMMUNOLOGY AND during inflammation. We used Ccr2-KO and Cx3cr1-KO mice to un- CCR2 crosstalk (7–9). Mice devoid of the Ccr2 gene exhibit derstand the role of these receptors in viral encephalitis-associated seizures and neurodegeneration, using the Theiler’s virus model of markedly reduced recruitment of monocytes and reduced pathology in several brain disease models, including autoimmune encephalitis, encephalitis in C57BL/6 mice. Our results show that CCR2 as well as – CX3CR1 plays a key role in the accumulation of myeloid cells in the multiple sclerosis, stroke, and status epilepticus (9 11). CNS and activation of hippocampal myeloid cells upon infection. Fur- Viral encephalitis is a frequent cause of early seizures, hippo- + − thermore, by using Cx3cr1-creER / tdTomatoSt/Wt reporter mice, we campal damage, and TLE, but the pathogenesis, mechanisms of show that, with regard to CD45 and CD11b expression, some micro- seizures, and hippocampal neurodegeneration after encephalitis glia become indistinguishable from monocytes during CNS infection. Interestingly, the lack of CCR2 or CX3CR1 receptors was associated Significance with almost complete prevention of hippocampal damage but did not prevent seizure development after viral CNS infection. These Viral encephalitis is a frequent medical emergency, often data are compatible with the hypothesis that CNS inflammatory resulting in acute seizures and brain damage, which reduce mechanism(s) other than the infiltrating myeloid cells trigger the de- quality of life, promote the development of epilepsy, and can velopment of seizures during viral encephalitis. cause death. The relative roles of activation of microglia, the brain-resident innate immune cells, versus invasion of blood- myeloid cells | epilepsy | Theiler’s virus | hippocampus | monocytes borne immune cells such as monocytes in the acute and chronic consequences of viral encephalitis are only incompletely un- emporal lobe epilepsy (TLE), the most frequent type of ep- derstood. Here we show that lack of the chemokine receptors Tilepsy in humans, is a common consequence of diverse brain CCR2 or CX3CR1, which regulate the responses of myeloid cells insults, including traumatic brain injury, stroke, and infections of such as monocytes and microglia, prevents hippocampal dam- the brain (1). Common to these different acute brain injuries is the age but not seizures in a mouse model of viral encephalitis. generation of an inflammatory response, which is thought to criti- Treatment strategies aimed at inhibiting peripheral immune cally contribute to the development of early (insult-associated) and cells from entering the brain during encephalitis could reduce late (spontaneous) seizures and neurodegeneration (1). A key brain damage. common brain response to injury is the rapid activation of micro- Author contributions: C.K., C.C., S.B., U.K., and W.L. designed research; C.K., C.C., I.W., glia, CNS-resident innate immune cells, which promptly release L.G., and I.G. performed research; C.K., C.C., I.W., L.G., U.K., and W.L. analyzed data; and inflammatory molecules including damage-associated molecular C.K., C.C., U.K., and W.L. wrote the paper. patterns, cytokines, and chemokines as well as related effector The authors declare no conflict of interest. pathways including cyclooxygenase-2 and complement factors (2, This article is a PNAS Direct Submission. 3).Furthermore,thereisincreasing evidence that inflammatory This open access article is distributed under Creative Commons Attribution-NonCommercial- monocytes invading from the periphery contribute significantly to NoDerivatives License 4.0 (CC BY-NC-ND). neuroinflammation and its devastating consequences after brain 1C.K., C.C., and S.B. contributed equally to this work. injury (4). However, the relative roles of invading monocytes and 2Present address: Neurona Therapeutics, South San Francisco, CA 94080. microglia for the short- and long-term consequences of acute brain 3To whom correspondence should be addressed. Email: wolfgang.loescher@tiho- injury are only incompletely understood. hannover.de. Monocytes, i.e., mononuclear phagocytes circulating in the This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. blood, normally do not invade the brain because of the blood– 1073/pnas.1806754115/-/DCSupplemental. brain barrier (BBB) (5). On the basis of their differential www.pnas.org/cgi/doi/10.1073/pnas.1806754115 PNAS Latest Articles | 1of10 Downloaded by guest on September 27, 2021 are only poorly understood (3). In a mouse model of viral tional experiments were performed in B6-based Cx3cr1-KO mice encephalitis-induced seizures and hippocampal damage, using to assess the role of CX3CR1 in myeloid cell activation in the intracerebral inoculation of Theiler’s virus [also termed “Theiler’s CNS during TMEV infection. In the brain, the fractalkine re- murine encephalomyelitis virus” (TMEV)] in C57BL/6J (B6) WT ceptor CX3CR1 is expressed primarily on brain-resident myeloid mice, two groups independently reported that brain-infiltrating cells, including microglia, and is critical in microglia activation inflammatory monocytes damage the hippocampus (12, 13) and and neuron–microglia communication (e.g., for recruitment of are key to the development of acute seizures (14). However, the microglia to injured neurons) (5). Thus, a comparison of the experimental methods used to analyze and reduce monocyte in- consequences of viral encephalitis in Ccr2-KO vs. Cx3cr1-KO vasion were not specific, so a role of other immune cells could not mice should allow conclusions about the relative roles of in- be excluded. Using a more selective approach for inhibiting filtrating monocytes and brain-resident myeloid cells, including monocyte invasion, i.e., administration of clodronate liposomes, microglia, in this model. we did not observe any prevention of hippocampal damage in this viral encephalitis model (15). Interestingly, in another mouse Results strain (SJL), in which infection with TMEV induces severe spinal Lack of CCR2, but Not of CX3CR1, Leads to Reduced Accumulation of cord demyelination, the use of Ccr2-KO mice reduced monocyte Myeloid Cells in the CNS After Intracerebral TMEV Infection. Because infiltration, demyelination, and long-term disease severity (16). CCR2 is implicated in the accumulation of infiltrating monocytes The objectives of the present study were threefold. The first in the CNS after infection (7), whereas CX3CR1 has been pro- aim was to better differentiate brain-resident myeloid cells, in- posed to be involved in microglia activation and neuron–micro- cluding microglia, from invading monocytes in the TMEV en- glia communication after CNS injury (5), we aimed to address cephalitis model of TLE. For this, we compared virus-induced the roles of CCR2 and CX3CR1 in viral encephalitis-induced + − effects in B6 WT vs. B6-based Cx3cr1-creER / tdTomatoSt/Wt hippocampal damage and seizure development. To this end we + mice in which long-lived CX3CR1 cells such as microglia can be first