Date Published: October 20, 2015
Publisher: Public Library of Science
Author(s): Matthew D. Martin, Marie T. Kim, Qiang Shan, Ramakrishna Sompallae, Hai-Hui Xue, John T. Harty, Vladimir P. Badovinac, Paul G. Thomas.
Memory CD8 T cells confer increased protection to immune hosts upon secondary viral, bacterial, and parasitic infections. The level of protection provided depends on the numbers, quality (functional ability), and location of memory CD8 T cells present at the time of infection. While primary memory CD8 T cells can be maintained for the life of the host, the full extent of phenotypic and functional changes that occur over time after initial antigen encounter remains poorly characterized. Here we show that critical properties of circulating primary memory CD8 T cells, including location, phenotype, cytokine production, maintenance, secondary proliferation, secondary memory generation potential, and mitochondrial function change with time after infection. Interestingly, phenotypic and functional alterations in the memory population are not due solely to shifts in the ratio of effector (CD62Llo) and central memory (CD62Lhi) cells, but also occur within defined CD62Lhi memory CD8 T cell subsets. CD62Lhi memory cells retain the ability to efficiently produce cytokines with time after infection. However, while it is was not formally tested whether changes in CD62Lhi memory CD8 T cells over time occur in a cell intrinsic manner or are due to selective death and/or survival, the gene expression profiles of CD62Lhi memory CD8 T cells change, phenotypic heterogeneity decreases, and mitochondrial function and proliferative capacity in either a lymphopenic environment or in response to antigen re-encounter increase with time. Importantly, and in accordance with their enhanced proliferative and metabolic capabilities, protection provided against chronic LCMV clone-13 infection increases over time for both circulating memory CD8 T cell populations and for CD62Lhi memory cells. Taken together, the data in this study reveal that memory CD8 T cells continue to change with time after infection and suggest that the outcome of vaccination strategies designed to elicit protective memory CD8 T cells using single or prime-boost immunizations depends upon the timing between antigen encounters.
Memory CD8 T cells provide immune hosts with enhanced protection from pathogenic infection due to an increased precursor frequency of antigen (Ag)-specific cells, widespread localization to both lymphoid and non-lymphoid tissues, and ability to rapidly execute effector functions such as cytokine production and cytolysis compared to naïve CD8 T cells [1–3]. Protection provided by memory CD8 T cells is dependent upon the number, quality (functional abilities), and location of memory CD8 T cells available at the time of infection. Importantly, the quality and location of memory CD8 T cells best suited to combat diverse infections is dependent upon the tropism of the invading pathogen. Memory CD8 T cells consist of a heterogeneous population of cells  that were initially categorized into central memory (Tcm) and effector memory (Tem) subsets based on CCR7 and CD62L expression, and that differ in anatomical location and functionality [5,6]. Recently, an additional subset of memory CD8 T cells has been described that reside in non-lymphoid tissues and that have been called tissue-resident memory (Trm) cells . While the relative protection provided by circulating Tcm and Tem cells differs depending on the nature of infection [6,8–10], both are better suited to provide protection against systemic infection than Trm cells that provide enhanced protection against infection that occurs within peripheral tissues [11–15]. Several studies have suggested that Trm cells may be long-lived in the skin following VacV or HSV infection and in mucosal surfaces following intramuscular immunization with adenovirus vectors [12,15,16]. However, other studies examining Trm generated following influenza have suggested that Trm cell numbers wane following infection . Therefore, longevity of Trm cells likely depends on the infection/vaccination model and the tissue of memory residence. However, circulating memory CD8 T cells persist for great lengths of time following immunization or systemic viral infection. For example, lymphocytic choriomeningitis virus (LCMV)-specific memory CD8 T cells are maintained at stable numbers in the spleen for the life of the laboratory mouse , and detectable numbers of memory CD8 T cells can be found in human PBL 20–75 years after natural exposure to, or vaccination against yellow fever virus, measles virus, and smallpox [19–23]. However, several studies have indicated that some properties of circulating memory CD8 T cells change with time after infection. For example, expression of CD62L and CD27 (markers of central memory cells) increases, indicating that the subset composition of the memory population changes with time after infection. In addition, functions such as cytokine production, proliferation, and memory generation following Ag re-encounter, increase with time [24–28]. The full extent of phenotypic and functional alterations that occur within the memory CD8 T cell population with time after infection, however, remains poorly characterized. It is unclear if alterations are due solely to differences in subset composition of memory CD8 T cell populations, or to changes within defined memory subsets. These are important questions to address, as the level of protection provided against systemic infections may change with time following initial infection and/or vaccination.
Protection provided by memory CD8 T cells is dependent upon their numbers, functional ability (quality), and location at the time of infection . We have shown that the quality of the circulating memory CD8 T cell population differs with time after infection in a manner not solely due to shifts in memory subset composition. Some functions of memory CD8 T cells analyzed on the population level, such as ability to produce IL-2, increased with time after infection, but were no different in CD62Lhi memory cells early or late after infection. However, other qualitative aspects of memory CD8 T cells including proliferation in response to the homeostatic cytokine IL-15 or to Ag, and mitochondrial function increased with time after infection when both the memory CD8 T cell population, and defined CD62Lhi subsets were analyzed. Thus, while some alterations in the functional abilities of memory CD8 T cells with time after infection can be attributed to shifts in subset composition, other qualitative changes cannot be wholly attributed to shifts in subset composition. Interestingly, as a consequence of these functional changes, the protection provided by memory CD8 T cell populations and CD62Lhi memory CD8 T cells against a chronic viral infection increased over time. Importantly, our data suggests that the outcome vaccination schemes designed to elicit protective memory CD8 T cells will depend on the timing between booster immunizations, and on the timing of re-infection following vaccination.