A timely immune response to pathogens relies on ability of a T cell to recognize an antigen and form immunological memory, allowing it to respond more rapidly upon reencounter of the same pathogen. Naïve T cells, or those that have yet to encounter a specific pathogen, respond much more slowly when encountering an antigen for the first time; however, a subset of CD8+ T cells possess memory-like qualities despite their naïve status. These “virtual memory” (TVM) cells arise by proliferation in response to lymphopenia early in neonatal development and share certain traits of “true memory” cells, including increased proliferation, inflammatory cytokine production, and increased expression of T cell activation markers. A distinct and stable population of TVM is found in mice, but the regulation of this population is unknown. To answer this question, Jessica Graham of the Lund lab (Vaccine and Infectious Disease Division) and colleagues hypothesized that Regulatory T cells (Tregs)—T cells that prevent immunopathology and autoimmunity by limiting effector T cell activation—might be responsible for maintenance of the TVM pool. They investigated the relationship between Tregs and TVM in a recent PNAS publication.
The authors began by establishing that TVM are subject to Treg-mediated control. When naïve mice were depleted of Tregs, TVM number and frequency increased drastically in both the blood and spleen four to six days post-depletion, suggesting that Tregs continuously exert suppressive control over the TVM population. Interestingly, although depleted Tregs repopulate within two weeks, the increase in TVM frequency persisted out to 28 days post-depletion, demonstrating that even a transient loss of Tregs leads to long-term increase in TVM. The authors then asked whether the increase in TVM was due to conversion of conventional naïve CD8+ T cells or the expansion of existing TVM. They transferred either naïve conventional CD8+ T cells or TVM into Treg-depleted mice and assessed the proliferation of both T cell population five days post-transfer. They found that new TVM only arose from existing TVM, not from conversion of conventional CD8+ T cells and that proliferating TVM retained their distinguishing phenotypes. Additionally, sustained Treg depletion did not lead to a further increase in TVM population, suggesting a finite frequency for this compartment. These studies confirmed that Tregs modulate TVM dynamics and limit the TVM population, prompting further questions about this interaction.
The authors then began to interrogate how Tregs might be restraining TVM. Interleukin-15 (IL-15) is a cytokine that is critical for T cell survival and function. T cells encounter IL-15 when it is trans-presented on dendritic cells, so Graham and colleagues hypothesized that Tregs might be restraining TVM indirectly by inhibiting transpresentation of IL-15. Accordingly, Treg depletion in mice led to an increase in both the frequency of IL-15-presenting dendritic cells and the amount of IL-15 presented on individual cells. Importantly, to directly link the requirement of IL-15 to TVM, the authors blocked IL-15 signaling in T cells, which led to a decrease in TVM. Together, these findings suggested that IL-15 transpresentation by dendritic cells is likely the target of Treg-mediated TVM control. Next, to understand the mechanism responsible for Treg-mediated inhibition of IL-15 transpresentation, the authors focused on CTLA-4, a Treg molecule that outcompetes effector T cells for binding with costimulatory markers on dendritic cells and therefore inhibits T cell activation. They found that mice with Treg-specific CTLA-4 knockout had increased TVM frequency compared to wildtype mice, suggesting that CTLA-4 is at least partly implicated in control of IL-15 transpresentation—and thus TVM —control, explaining one mechanism by which Tregs restrain TVM.
Once establishing how Tregs limit TVM development, the authors sought to understand what purpose Treg-mediated TVM inhibition at baseline serves during subsequent viral infection. To test the consequences of unchecked TVM proliferation, they used conditional knockout mice whose Tregs lack integrin b1 (a component of VLA-4 integrin that allows sustained interaction between Tregs and dendritic cells) as a model of TVM-enriched mice, as Tregs lacking VLA-4 are unable to stably interact with dendritic cells and inhibit TVM. The authors then compared disease outcomes between TVM-enriched and wildtype mice infected and later challenged with West Nile virus (WNV). They found that TVM-enriched mice had an increased number of total CD8+ T cells six days post-secondary infection, but the number of WNV-specific T cells was decreased compared to wildtype. Furthermore, TVM -enriched mice had more severe clinical scores, earlier death, and higher WNV loads in the brain, demonstrating that early TVM proliferation may fill the CD8+ T cell niche, preventing expansion of virus-specific T cells upon infection. Therefore, these results highlight a role for Tregs in which pre-emptive control of TVM populations allows for more robust development of WNV-specific T cells during a memory response to viral pathogen.
Graham explained that their study “points to a critical role for Tregs in regulating both IL-15 trans-presentation as well as unconventional memory T cell development, with downstream implications for preserving the ability to recall effective memory T cells upon future pathogen encounter.” Their findings suggest that “CD8+ T cell access to IL-15 over the course of a viral infection, and in particular WNV infection, is a defining component of host protection and that introducing additional competition in the form of TVM for this resource in an environment of virally imposed scarcity tips the balance further in favor of WNV.” However, this novel relationship between Tregs and TVM has relevance outside WNV; these results have “critical implications for Treg-directed therapeutics, such as CTLA-4 and VLA-4 blockades, as there is potential for secondary long-term effects on the compositional dynamics of the host CD8+ T cell population, such as decreased T cell responsiveness to vaccination and subsequent pathogen exposures.”
This work was supported by the National Institutes of Allergy and Infectious Diseases, the National Institutes of Health, and the Diseases of Public Importance Training Grant.
UW/Fred Hutch Cancer Consortium member Jennifer Lund contributed to this work.
Da Costa AS*, Graham JB*, Swarts JL, Lund JM. 2019. Regulatory T cells limit unconventional memory to preserve the capacity to mount protective CD8 memory responses to pathogens. PNAS. DOI: 10.1073/pnas.1818327116
*These authors contributed equally to this work.