Latent herpesvirus infection in human trigeminal ganglia causes chronic immune response

Abstract

The majority of trigeminal ganglia (TGs) are latently infected with alpha-herpesviruses [herpes simplex virus type-1 (HSV-1) and varicella-zoster virus (VZV)]. Whereas HSV-1 periodically reactivates in the TGs, VZV reactivates very rarely. The goal of this study was to determine whether herpesvirus latency is linked to a local immune cell infiltration in human TGs. T cells positive for the CD3 and CD8 markers, and CD68-positive macrophages were found in 30 of 42 examined TGs from 21 healthy individuals. The presence of immune cells correlated constantly with the occurrence of the HSV-1 latency-associated transcript (LAT) and only irregularly with the presence of latent VZV protein. In contrast, uninfected TGs showed no immune cell infiltration. Quantitative RT-PCR revealed that CD8, interferon-gamma, tumor necrosis factor-alpha, IP-10, and RANTES transcripts were significantly induced in TGs latently infected with HSV-1 but not in uninfected TGs. The persisting lymphocytic cell infiltration and the elevated CD8 and cytokine/chemokine expression in the TGs demonstrate for the first time that latent herpesviral infection in humans is accompanied by a chronic inflammatory process at an immunoprivileged site but without any neuronal destruction. The chronic immune response seems to maintain viral latency and influence viral reactivation.

Figure 1.

Representative photomicrographs of tissue sections from human TGs latently infected with HSV-1 and/or VZV, which were stained by IHC with anti-CD3, anti-CD8, and anti-CD68 antibodies. Neuron encircled by several rows of CD3-positive T cells (arrow, a) and clusters of CD3-positive T cells among ganglionic neurons (arrow, b). Infiltrating T cells stained positive with anti-CD8 antibodies. The pattern of staining was similar to that of CD3-positive T cells. CD8-positive T cells surrounding single neurons (arrow, c; encircled, d). Only a few T cells stained positive with anti-CD4 antibodies (encircled, e; arrowheads point to CD4-positive cells scattered among the ganglionic neurons). T cells were also present as clusters among nerve fibers: CD3-positive cells in a sagittal section (encircled, f) and CD8-positive cells in a transverse section of nerve fibers (encircled, g). When LAT ISH was combined with IHC (h) some CD3-positive T cells (arrowheads) were found occasionally around LAT-positive neurons (arrows). Single cells or clusters of cells among the neurons showed CD68 positivity (encircled, i). Small clusters of CD68-positive macrophages (encircled, j) were found in the vicinity of LAT-positive neurons (arrows). Neurons positive for VZV-protein 62 (k, arrowheads) from a child who died of sudden infant death and who showed no HSV-1 and no T-cell infiltration. Neurons positive for VZV-protein 62 (l, filled arrowheads) from a young man who was latently infected with HSV-1 and showed T-cell infiltrates (arrows, l) around VZV-free neurons. Some neurons showed brown lipofuscin granules near the plasma membrane (open arrowheads, l) distinguishable from the VZV protein that usually fills up the whole cytoplasm. Photomicrographs of a to c, f, and h to l were taken from paraffin and photomicrographs of d, e, g from frozen tissue sections. Magnification, ×400. Tissue sections from a, b, d, e, g, k, and l were slightly counterstained with hematoxylin.

Figure 2.

Quantification of CD8, cytokine, and chemokine mRNA expression by real-time RT-PCR. Each diagram shows the comparison of 12 HSV-1 negative TGs (neg) with 15 HSV-1 positive TGs (pos). The y axis represents the relative transcript number of the gene of interest compared to the housekeeping gene. Dots represent mean values, boxes represent SEM, and whiskers represent 1.96 × SEM.