Herpes Simplex Virus 1, Macrophages, and the Cornea

LETTER

The maintenance of the pristine structure and function of the cornea is imperative to preserve the visual axis. Herpes simplex virus 1 (HSV-1) is a common human pathogen that can render havoc to the infected cornea, resulting in neovascularization and opacity that ultimately compromises visual acuity (1, 2). The identification and characterization of components of the immune system that safeguard the integrity of the anterior segment of the eye, including the cornea, in response to infection or transplantation are major quests for ocular immunologists.

A recent Journal of Virology article by Lee and Ghiasi (3) proposed to explore the role of “M1” and “M2” macrophages in HSV-1 infection of the cornea and trigeminal ganglion by using mice transduced intramuscularly with plasmid constructs containing genes that “shift” the immune response toward an M1 or M2 macrophage preference. The authors suggest that mice transduced with a gamma interferon (IFN-γ)-expressing plasmid were found to shed more virus in the tears following HSV-1 infection than plasmid vector-controlled transduced mice, whereas mice transduced with a CSF-1-expressing plasmid were found to shed less virus than controls. Consistent with these results, less latent virus was found in the trigeminal ganglia of the CSF-1 plasmid-transduced mice than in vector or IFN-γ plasmid-transduced animals. The authors conclude that CSF-1 may be a useful adjuvant as part of an anti-HSV-1 vaccine.

Whereas the topic is of considerable interest, there are a number of concerns about the study and inaccurate statements included in the paper. The opening statement in the abstract, indicating that macrophages are the predominant infiltrate in the corneas of mice ocularly infected with HSV-1, is not correct. Neutrophils have consistently been found to be the dominant immune cell infiltrate in the cornea following HSV-1 infection (4, 5) out into latency (6). The authors go on to state that “resident macrophages are not detectable in the corneas of naive mice.” This is also incorrect, as several published studies (reviewed in references 7 and 8), including one inaccurately cited in reference 3 (9), would refute this statement. Yet, another generalized statement included in the introduction suggests that “the least efficacious vaccines are associated with the persistence of very high levels of infiltrating macrophages” (citing their own work). Our lab has recently published a vaccine against ocular HSV-1 infection that we submit is the most efficacious to date in mice blocking local viral replication and establishment of latency while retaining corneal function with elevated levels of macrophages (F4/80⁺ Gr-1⁻) found in the cornea relative to naive mice (10).

There are other troubling features in this article, including the lack of evidence that the transgenes administered into the muscle of mice are actually expressed and how macrophages found in the spleen or peritoneum of the transduced mice influence the local immune response in the cornea and trigeminal ganglion in suppressing virus replication. Conclusions drawn from data included in the absence of statistical analysis (i.e., Tables 1 and 2) and inappropriate statistical analysis (e.g., group sizes larger than two analyzed in the absence of analysis of variance [Fig. 8 and 9]) are also disconcerting.