Non-traditional signaling: Memory T cells activate innate immunity

From the Zhu and Corey Lab, Vaccine and Infectious Disease Division

How does our immune system defend us against viruses at sites of recurrent infection? This fundamental question is one driver of the research conducted by Drs. Tao Peng, Jia Zhu, and Larry Corey in the Zhu and Corey Lab with the aim of applying this basic knowledge to vaccine development. They recently published their findings in the journal of Frontiers in Immunology.  Corresponding author Dr. Zhu described the research as “an in vivo interaction between the adaptive and innate/intrinsic immune system in naturally recurring herpes simplex virus type 2 (HSV-2) infection in human skin.”  

The innate and adaptive immune systems function together to sense pathogens, signal accordingly, and restrict pathogen expansion in the host. Traditionally, these systems coordinate antiviral responses by first activating the innate immune system leading to a subsequent trigger of the adaptive immune response. The researchers used sequential biopsies of recurrent HSV-2 infection in humans to ask questions about local immune signaling. Intriguingly, they characterized a feedback loop in which the adaptive immune cells activate innate immune signaling in neighboring cells to locally protect against herpesvirus replication and herpetic lesion formation.

This critical finding stemmed from the researchers’ interest in understanding the mechanism of how tissue-resident-memory T cells (TRM) contribute to the antiviral response to HSV-2 infection. As the name suggests, TRM cells do not recirculate but rather remain long term at epithelial barrier tissues, which include the skin as well as the gastrointestinal tract, reproductive tract, and lungs. Following a primary HSV infection, TRM transit to sites of infection and remain there even after the virus has been cleared. The CD8+ TRM (CD8TRM) cells bind HSV-2 peptide, suggesting a role in advancing virus-specific adaptive immune responses. Yet, the adaptive antiviral response does not explain the rapid response observed to restrict herpesvirus recurrent activation and local spread. To better understand the specific mechanism of how CD8TRM enhance the antiviral response to local recurrent HSV-2 infection, the researchers performed laser capture microdissection and transcriptional profiling analyses on patient skin biopsies of acute and healed HSV-2 lesions to characterize interactions between CD8TRM cells and proximal keratinocytes and epithelial cells. This transcriptional survey revealed downregulation of genes involved in transcriptional machinery and upregulation of many genes related to interferon antiviral activities in keratinocytes. From these findings the researchers proposed that CD8TRM cells may activate innate signaling and restrict HSV-2 gene transcription in neighboring cells. 

Higher HSV replication occurs in keratinocytes and epithelial cells for microenvironments with limited IFN-ɣ (Susceptible). Restricted HSV replication occurs in keratinocytes and epithelial cells for microenvironments in which CD8TRM cells highly express IFN-ɣ (Resistance).
Higher HSV replication occurs in keratinocytes and epithelial cells for microenvironments with limited IFN-ɣ (Susceptible). Restricted HSV replication occurs in keratinocytes and epithelial cells for microenvironments in which CD8TRM cells highly express IFN-ɣ (Resistance). Image provided by Dr. Zhu

Upon further investigation of the interferon response in HSV-2 lesions, the researchers discovered that IFN-ɣ was more abundant than other interferons and intriguingly, was primarily produced by CD8TRM cells in HSV-2 lesions. IFN-ɣ is known to activate and enhance the adaptive immune response. In addition to this role, the researchers tested and confirmed that IFN-ɣ treatment of keratinocytes also upregulated antiviral gene expression supporting a function of CD8TRM expression of IFN-ɣ in activating the innate immune response. To determine if IFN-ɣ secreted from CD8TRM resulted in antiviral innate immune activation in neighboring keratinocytes and epithelial cells, skin biopsy tissues were analyzed for concurrent expression of IFN-ɣ and IFN-ɣ mediated antiviral (IMA) gene expression in cells of the lesion site microenvironment. Healed lesion sites with high IFN-ɣ expression had 2-fold higher expression of IMA as compared to healed lesion sites with low IFN-ɣ expression. These findings support the researchers’ hypothesis that higher IFN-ɣ levels correlate with increased antiviral gene expression in neighboring cells in the microenvironment of healed HSV-2 lesion sites.

As mentioned previously, the researchers discovered that keratinocytes within HSV-2 lesions had reduced expression of genes involved in transcriptional activity as compared to control biopsy tissues. They predicted that high IFN-ɣ levels may also contribute to transcriptional repression of HSV-2 transcription. This hypothesis was confirmed by showing that pretreatment of human keratinocytes or fibroblast cells with IFN-ɣ resulted in greater than 100-fold reduction in transcription for most immediate-early HSV-2 genes. Furthermore, IFN-ɣ pretreatment delayed virus replication by 6-8 hours and reduced the total viral yield for >100 folds when tested in cell culture systems. Importantly, this slowed virus replication in IFN-ɣ pretreated cells would allow innate immune responses to greatly stunt virus production and disease severity.  

This publication provided insight into the mechanism of how TRM cells confer local tissue-based immunity and pathogen containment. Dr. Zhu stated, “our research indicates that TRM accomplish these tasks [of tissue-based immunity and pathogen containment] by communicating with their neighboring epithelial cells to significantly enhance their intrinsic antiviral activities and interferon-associated pathways. This communication markedly amplifies the “antiviral shield” of the surrounding area, making them highly resistant to viral infection.” To extend this research, Dr. Zhu commented on their interest in studying the correlation between the IFN-ɣ-dependent antiviral gene expression and disease severity for genital herpes infections. Additionally, future work may also extend to the roles of CD8TRM:epithelial cell crosstalk during SARS-CoV-2 infection in the lungs and how this adaptive to innate immune signaling impacts COVID-19 disease outcomes. 

The spotlighted research was funded by the National Institutes of Health.

UW/Fred Hutch Cancer Consortium members Keith Jerome, David Koelle, Anna Wald, Larry Corey, and Jia Zhu contributed to this work.

Peng T, Phasouk K, Sodroski CN, Sun S, Hwangbo Y, Layton ED, Jin L, Klock A, Diem K, Magaret AS, Jing L, Laing K, Li A, Huang ML, Mertens M, Johnston C, Jerome KR, Koelle DM, Wald A, Knipe DM, Corey L, Zhu J. 2021. Tissue-Resident-Memory CD8+ T Cells Bridge Innate Immune Responses in Neighboring Epithelial Cells to Control Human Genital Herpes. Front Immunol. 12:735643.