Abstract

Memory CD8+ T cells represent an important effector arm of the immune response in maintaining long-lived protective immunity against viruses and some intracellular bacteria such as Listeria monocytogenes (L.m). Memory CD8+ T cells are endowed with enhanced antimicrobial effector functions that perfectly tail them to rapidly eradicate invading pathogens. It is largely accepted that these functions are sufficient to explain how memory CD8+ T cells can mediate rapid protection. However, it is important to point out that such improved functional features would be useless if memory cells were unable to rapidly find the pathogen loaded/infected cells within the infected organ. Growing evidences suggest that the anatomy of secondary lymphoid organs (SLOs) fosters the cellular interactions required to initiate naive adaptive immune responses. However, very little is known on how the SLOs structures regulate memory immune responses. Using Listeria monocytogenes (L.m) as a murine infection model and imaging techniques, we have investigated if and how the architecture of the spleen plays a role in the reactivation of memory CD8+ T cells and the subsequent control of L.m growth. We observed that in the mouse, memory CD8+ T cells start to control L.m burden 6 hours after the challenge infection. At this very early time point, L.m-specific and non-specific memory CD8+ T cells localize in the splenic red pulp and form clusters around L.m infected cells while naïve CD8+ T cells remain in the white pulp. Within these clusters that only last few hours, memory CD8+ T produce inflammatory cytokines such as IFN-γ and CCL3 nearby infected myeloid cells known to be crucial for L.m killing. Altogether, we describe how memory CD8+ T cells trafficking properties and the splenic micro-anatomy conjugate to create a spatio-temporal window during which memory CD8+ T cells provide a local response by secreting effector molecules around infected cells.

Highlights

  • The protective immune response against pathogenic microorganisms involves two components: a rapid, antigen non-specific innate response and a delayed, acquired response specific for the antigens (Ags) displayed by invading microbes

  • Fibroblastic Reticular Cells (FRCs) residing in the T cell zones of secondary lymphoid organs (SLOs) dictate the migration and define the territory of naıve T lymphocytes in these organs [19]

  • Data reveal that the density of memory OT-I cells in the RP was two times higher than in the WP, indicating that memory OT-I cells predominantly colonize the RP of the spleen, correlating with the observation that 2/3 of the OT-I isolated from the same spleen belong to the TEM subset (CD62Llow CCR7low, not shown)

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Summary

Introduction

The protective immune response against pathogenic microorganisms involves two components: a rapid, antigen non-specific innate response and a delayed, acquired response specific for the antigens (Ags) displayed by invading microbes The efficacy of these responses relies on multiple effector functions such as the secretion of antimicrobial molecules and the direct killing of infected cells by effector lymphocytes [1]. While the ability of a lymphocyte to visit all LNs is critical for a thorough monitoring of potential infections, it can be viewed as distracting the cell from where it is mostly needed upon infection namely the SLO draining the infected site This notion is crucial since lymphocytes wander approximately 12 hours in a given SLO before they can return to the blood circulation and get another opportunity to home to the SLO draining this inflamed tissue [3]

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