Date Published: April 18, 2019
Publisher: Public Library of Science
Author(s): Dhong Hyun Lee, Ujjaldeep Jaggi, Homayon Ghiasi, Susmit Suvas.
Complex interactions between HSV-1 and infiltrating immune cells play important roles in establishing localized, acute virus replication as well as chronic latent infection. The extent and duration of initial virus replication are the key determinants of subsequent pathologic inflammatory responses and therefore, the accumulation of immune cell populations at this time point is a key target for prevention. Therefore, we evaluated the role of various immune cell infiltrates between 1 h and 28 days post-infection (PI) using mice infected with virulent HSV-1 strain McKrae without corneal scarification. The effect of corneal scarification on immune cell infiltrates was also determined. We first determined the activation status and origin of macrophage infiltrates as early as 1 h PI. We found a sharp increase in the total macrophage population after 12 h PI, that was primarily due to infiltration of CCR2+ migratory macrophages, mostly in M1 status (MHC II+). The number of CCR2- resident macrophages, mostly unpolarized (M0), increased gradually over time and peaked at 48 h PI. Interestingly, some of the resident macrophages gained an M2-like phenotype (CD206Low), which peaked at 12 h PI, concurrent with M1 macrophage infiltration. From 1–7 days PI, infiltration of various immune cells correlated strongly with HSV-1 replication, with neutrophils showing the biggest increase, and NKT cells the biggest decrease, after infection. The presence of geographical ulcer did not correlate with increased infiltration, while mice with corneal scarring had significantly more immune cell infiltration than those without corneal scarring. Overall, we showed time-dependent infiltration of various immune cells in the eye of HSV-1 infected mice. Initial infiltration of macrophages followed by infiltration of T cells at later times PI demonstrates the importance of targeting macrophages rather than other immune cells type, for therapeutic treatment of HSV-1.
It is well known that herpes stromal keratitis (HSK) mediated by herpes simplex virus type 1 (HSV-1) is an immunopathological disease and that immune cells play important roles in clearing the virus from the eye around days 6–7 post-infection (PI) . HSK is the most common cause of vision impairment in humans, and occurs as a consequence of virus reactivation . The extent and duration of immune cell infiltrates in the eye during both primary HSV-1 infection and reactivation can impact the severity of eye disease and the subsequent HSK, also is known as corneal scarring (CS) [3–11]. After ocular HSV-1 infection, innate immune cells are thought to play an important role in clearing virus from the eye. Recent studies showed that neutrophils, which start their response around 18 h PI, peak at day 2 PI, and eventually decline , along with other innate immune cells including NK cells, γ-delta T cells, macrophages, and dendritic cells (DCs), participate in virus clearance [13, 14].
HSK is the predominant cause of human blindness in the western world. To understand the pathology of the disease and to develop prevention strategies, it is essential to investigate the effect of HSK on the host immune response. It has been reported that higher numbers of immune cells in the eye correlate with increased pathogenesis [15, 17]. Virus strain, infection dose, mouse strain, and physical alteration of corneas affect HSV-1 pathogenesis. Initial viral replication in epithelial cells of the cornea begins with virus entry, which has detrimental effects on the host immune system. Innate immune cells play an important role in viral clearance, which is sustained by the adaptive immune response . In humans, HSV-1 infections are difficult to study because primary infections are usually asymptomatic. Therefore, mouse models are an excellent way to study HSV-1 induced pathogenesis . Recent studies have shown that HSK pathogenesis depends largely on immune responses generated by the host, but also depends on important factors like virus strain, viral clearance, and mice strain .
Effective therapies for HSV-1 infection in the eye are lacking, despite knowledge that various immune cell infiltrates are involved in protection and disease following ocular HSV-1 infection. We identified various immune cell infiltrates in the eye of ocularly-infected mice and their presence was time dependent. Among these infiltrates, macrophages accumulated in the eye of HSV-1 infected mice as early as 1 h PI, thus modulating macrophage activity could be an effective immunotherapeutic approach to control HSV-1 associated eye disease. We also found that corneal scarification significantly affected the number and type of infiltrating immune cells. Therefore, data generated with corneal scarification prior to HSV-1 infection should be interpreted with caution.