Mice primed with influenza virus and then challenged by injection of a neurotropic strain of the virus into a cerebral ventricle showed massive recruitment of memory T cells into the brain which rescued the animals from fatal encephalitis 16. Strikingly, the numbers of activated, influenza-specific CD8+ T cells

within the brain remained elevated for a year in the absence of clear evidence of persisting influenza antigen. Given the known isolation of the CNS from the recirculating pool lymphocytes, this finding suggested the long-term residence of memory T cells at this site. In a simple but informative experiment, Klonowski et al. 17 joined the circulation of pairs of congenically marked mice by parabiosis to examine the dynamics of memory T-cell trafficking. They reported that while memory cells

in most tissues and selleck inhibitor organs equilibrated with kinetics similar to the mixing of the bloodstreams, memory CD8+ T cells in the brain and intestinal mucosa of partner mice did not equilibrate. Further evidence that memory CD8+ T cells in the CNS are separated from the recirculating memory pools was presented by Wei et al. 18, who showed that peptide injection could not delete memory T cells in the brain although memory cells in all other tissues were deleted. Intranasal infection with vesicular stomatitis virus (VSV)

LDE225 datasheet not only results in respiratory tract infection, but also allows the virus to spread to the brain via the olfactory nasal epithelium and its connection to the olfactory bulb 19. Following infection via the nares, we observed “hot spots” of VSV infection throughout the brain early after infection 20. Virus-specific CD8+ T cells flooded into the brain after being activated in peripheral lymphoid organs, swarmed around the VSV-infected hot spots and cleared the infection Exoribonuclease by 8 days. Numbers of CD8+ T cells in the brain plunged thereafter but a fraction remained in the brain for months and these resident lymphocytes were grouped into clusters in the brain parenchyma, presumably at the previous hot spots of infection. These memory CD8+ T cells did not mix with the circulation, and were unique in their high expression of CD103 and low level expression of CD122. Upregulation of CD103 was absolutely dependent on the T cells interacting with their antigen in the brain. On-site recognition of viral antigen and CD103 expression determined, to a great extent, the number of virus-specific memory cells that remained in the CNS. In these experiments, viral antigen or viral genomic RNA could not be detected in the CNS memory T-cell clusters.