Macrophages From Latently Infected Mice Have Trained Immunity to HSV-1

Our previous studies demonstrated that macrophages play a crucial role in both primary and latent herpes simplex virus 1 (HSV-1) infections. Here, we sought to determine whether HSV-1 exposure induces long-lasting functional and epigenetic changes in macrophages consistent with trained immunity, leading to enhanced responses upon secondary stimulation.

To explore this, we performed Assay for Transposase-Accessible Chromatin sequencing (ATAC-seq) analysis on isolated spleen- and bone marrow (BM)-derived macrophages from latently infected mice before and after stimulation with UV-inactivated virus to identify open chromatin regions indicative of changes in gene regulation. Additionally, we performed flow cytometric analysis of infected spleen macrophages, BM-derived macrophages, corneas, and the trigeminal ganglia (TG). Moreover, to assess the durability of training response to infection, we evaluated responses after secondary infection.

The study revealed that immunity-related GTPase family M protein (IRGM1) expression in isolated macrophages from latently infected mice was significantly elevated after stimulation, compared with that of more than 900 genes with open or closed chromatin accessibility. Flow cytometry further confirmed a higher proportion of IRGM1+ macrophages in the spleen, BM, cornea, and the TG of latently infected mice compared with mock-infected controls. The qRT-PCR determined that macrophages isolated from the spleen, trigeminal ganglia, and BM of latently infected mice continued to exhibit elevated IRGM1 expression levels relative to controls.

Collectively, our findings indicate that macrophages develop a durable trained immunity to HSV-1 infection, with IRGM1 emerging as a key component in the long-term maintenance of macrophage immunological memory.