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The chemokine CXCL10 is crucial for the control of viral replication through the regulation of mobilization of antigen-specific T cells to sites of infection. CXCL10 is highly expressed both at sites of inflammation as well as constitutively within lymphoid organs by both bone marrow (BM)-derived and non-BM-derived cells. However, the relative immunologic importance of CXCL10 expressed by these divergent sources relative to HSV-1 infection is unknown. Using mouse chimeras reconstituted with either wild type or CXCL10 deficient mouse BM, we show BM-derived, radiation-sensitive cells from wild type mice were solely responsible for resistance to HSV-1 in the trigeminal ganglia and brain stem. The resistance was not reflected by a deficiency in the recruitment of effector cells to sites of inflammation or expression of chemokines or IFN-gamma and likely results from additional, yet-to-be-determined factors emanating from wild type, BM-derived cells.

The chemokine, CXCL10, chemotactic for NK cells, activated T cells, and dendritic cells is highly expressed during viral infections, including HSV-1. The importance of this chemokine to the control of HSV-1 infection was tested using mice deficient in CXCL10 (CXCL10−/−). Following corneal infection, HSV-1 viral titers were elevated in the nervous system of CXCL10−/− mice, which correlated with defects in leukocyte recruitment including dendritic cells, NK cells, and HSV-1-specific CD8+ T cells to the brain stem. In the absence of NK cells and HSV-1-specific CD8+ T cells in wild-type (WT) or CXCL10−/− mice, similar levels of virus were recovered in the nervous system, suggesting these cells are responsible for the observed defects in the control of viral replication in CXCL10−/− mice. Leukocyte mobilization was also compared between WT, CXCL10−/−, and mice deficient in the only known receptor for CXCL10, CXCR3 (CXCR3 −/−). NK cell mobilization was comparably reduced in both CXCL10−/− and CXCR3−/− mice relative to WT animals. However, the reduction in mobilization of HSV-1-specific CD8+ T cells in CXCL10−/− was not observed in CXCR3−/− mice following HSV-1 infection. The defect was not the result of an alternative receptor for CXCL10, as Ag-specific CD8+ T cell recruitment was not reduced in mice which were deficient in both CXCL10 and CXCR3. Thus, CXCL10 deficiency results in reduced mobilization of HSV-1-specific CD8+ T cells as a result of dysregulation of CXCR3 signaling.

Hematopoietic stem and progenitor cells were previously found to express Toll-like receptors (TLRs), suggesting that bacterial/viral products may influence blood cell formation. We now show that common lymphoid progenitors (CLPs) from mice with active HSV-1 infection are biased to dendritic cell (DC) differentiation, and the phenomenon is largely TLR9 dependent. Similarly, CLPs from mice treated with the TLR9 ligand CpG ODN had little ability to generate CD19+ B lineage cells and had augmented competence to generate DCs. TNFα mediates the depletion of late-stage lymphoid progenitors from bone marrow in many inflammatory conditions, but redirection of lymphopoiesis occurred in TNFα−/− mice treated with CpG ODN. Increased numbers of DCs with a lymphoid past were identified in Ig gene recombination substrate reporter mice treated with CpG ODN. TLR9 is highly expressed on lymphoid progenitors, and culture studies revealed that those receptors, rather than inflammatory cytokines, accounted for the production of several types of functional DCs. Common myeloid progenitors are normally a good source of DCs, but this potential was reduced by TLR9 ligation. Thus, alternate differentiation pathways may be used to produce innate effector cells in health and disease.

The role of CXCL9 and CXCL10 in the ocular immune response to herpes simplex virus type 1 (HSV-1) infection was investigated using mice deficient in either CXCL9 or CXCL10. CXCL10 but not CXCL9 deficient mice showed an increase in sensitivity to ocular virus infection as measured by an elevation in virus titer recovered in the tear film and corneal tissue. The increase in virus was associated with an increase in the expression of the chemokine CCL2 but no significant change in the infiltration of CD4 + T cells or NK cells into the corneal stroma. In contrast, a significant reduction in CD4 + T cell infiltration into the cornea was found in CXCL9 deficient mice following HSV-1 infection consistent with the absence of CXCL9 expression and reduction in expression of other chemokines including CCL3, CCL5, CXCL1, and CXCL10. Collectively, the results suggest a non-redundant role for CXCL9 and CXCL10 in response to ocular HSV-1 infection in terms of controlling virus replication and recruitment of CD4 + T cells into the cornea.

Herpes simplex virus-1 (HSV-1) causes lifelong infection affecting between 50 and 90% of the global population. In addition to causing dermal lesions, HSV-1 is a leading cause of blindness resulting from recurrent corneal infection. Corneal disease is characterized by loss of corneal immunologic privilege and extensive neovascularization driven by vascular endothelial growth factor-A (VEGF-A). In the current study, we identify HSV-1 infected cells as the dominant source of VEGF-A during acute infection, and VEGF-A transcription did not require TLR signaling or MAP kinase activation. Rather than being an innate response to the pathogen, VEGF-A transcription was directly activated by the HSV-1 encoded immediate early transcription factor, ICP4. ICP4 bound the proximal human VEGF-A promoter and was sufficient to promote transcription. Transcriptional activation also required cis GC-box elements common to the VEGF-A promoter and HSV-1 early genes. Our results suggest that the neovascularization characteristic of ocular HSV-1 disease is a direct result of HSV-1's major transcriptional regulator, ICP4, and similarities between the VEGF-A promoter and those of HSV-1 early genes., Author Summary Herpes simplex virus-type 1 is the leading cause of infectious corneal blindness in the industrialized world. Most of the morbidity associated with the virus is due to the host response to episodic reactivation of latent virus. Corneal immunologic privilege is associated with a number of factors including the absence of blood and lymphatic vessels. Conversely, corneal hem (blood)- and lymph-angiogenesis driven by inflammation correlate with the loss of privilege. Neovascularization is a common phenomenon in HSV-1 keratitis that correlates with poor prognosis. We have previously discovered HSV-1 elicits corneal lymphangiogenesis through a unique mechanism involving vascular endothelial growth factor (VEGF)-A independent of that described for other insults including transplantation or bacterial infection. However, the viral-encoded product(s) that elicit host production of VEGF-A is(are) unknown. In this paper, we have identified infected cell protein-4 (ICP4) as the primary virus-encoded product that drives VEGF-A expression. As VEGF-A is involved in driving neovascularization associated with tumor growth and metastasis, proteins that influence transcriptional regulation of VEGF-A may be useful in the development of adjunct therapy for such disparate diseases as cancer and HSV-1 keratitis.

Inflammatory lymphangiogenesis plays a crucial role in the development of inflammation and transplant rejection. The mechanisms of inflammatory lymphangiogenesis during bacterial infection, toll-like receptor ligand administration, and wound healing are well characterized and depend on ligands for the vascular endothelial grow factor receptor (VEGFR) 3 that are produced by infiltrating macrophages. But inflammatory lymphangiogenesis in nonlymphoid tissues during chronic viral infection is unstudied. Herpes simplex virus 1 (HSV-1) infection of the cornea is a leading cause of blindness and depends on aberrant host immune responses to antigen within the normally immunologically privileged cornea. We report that corneal HSV-1 infection drives lymphangiogenesis and that corneal lymphatics persist past the resolution of infection. The mechanism of HSV-1–induced lymphangiogenesis was distinct from the described mechanisms of inflammatory lymphangiogenesis. HSV-1–elicited lymphangiogenesis was strictly dependent on VEGF-A/VEGFR-2 signaling but not on VEGFR-3 ligands. Macrophages played no role in the induction of lymphangiogenesis and were not a detectable source of VEGF-A. Rather, using VEGF-A reporter transgenic mice, we have identified infected epithelial cells as the primary source of VEGF-A during HSV-1 infection. Our results indicate that HSV-1 directly induces vascularization of the cornea through up-regulation of VEGF-A expression.

Type I IFNs are potent antiviral cytokines that contribute to the development of the adaptive immune response. To determine the role of type I IFNs in this process in an infectious disease model, mice deficient in the type I IFN receptor (CD118−/−) were ocularly infected with HSV-1 and surveyed at times post infection in the nervous system and lymph node for virus and the host immune response. Virus titers were elevated in the trigeminal ganglia and brain stem with virus disseminating rapidly to the draining lymph node of CD118−/− mice. T cell and plasmacytoid dendritic cell infiltration into the brain stem was reduced in CD118−/− mice following infection, which correlated with a reduction in CXCL10 but not CXCL9 expression. In contrast, CXCL1 and CCL2 levels were up-regulated in the brainstem of CD118−/− mice associated with an increase in F4/80+ macrophages. By day 5 post infection, there was a significant loss in T, NK, and plasmacytoid dendritic cell numbers in the draining lymph nodes associated with an increase in apoptotic/necrotic T cells and an appreciable lack of HSV-specific CD8+ T cells. The adoptive transfer of HSV-specific TCR transgenic CD8+ T cells into CD118−/− mice at the time of infection modestly reduced viral titers in the nervous system suggesting in addition to the generation of HSV-specific CD8+ T cells, other type I IFN-activated pathways are instrumental in controlling acute infection.

In response to ocular herpes simplex virus type 1 (HSV-1) infection in mice, a rapid induction or increase in the local expression of chemokines, including CXCL10, is found. The present study investigated the role of the receptor for CXCL10, CXCR3, in the host response to corneal HSV-1 infection. Mice deficient in CXCR3 (CXCR3(-/-)) were found to have an increase in infectious virus in the anterior segment of the eye by day 7 postinfection. Coinciding with the increase, selective chemokines, including CCL2, CCL3, CCL5, CXCL9, and CXCL10, were elevated in the anterior segment of the HSV-1-infected CXCR3(-/-) mice. In contrast, there was a time-dependent reduction in the recruitment of natural killer (NK) cells (NK1.1(+)CD3(-)) into the anterior segment of CXCR3(-/-) mice. A reduction in NK cells residing in the anterior segment of mice following antiasialoGM1 antibody treatment resulted in an increase in infectious virus. No other leukocyte populations infiltrating the tissue were modified in the absence of CXCR3. Collectively, the loss of CXCR3 expression specifically reduces NK cell mobilization into the cornea in response to HSV-1.

Mice deficient in RNA-dependent protein kinase (PKR -/- ) or deficient in PKR and a functional 2',5'-oligoadenylate synthetase (OAS) pathway (PKR/RL -/- ) are more susceptible to genital herpes simplex virus type 2 (HSV-2) infection than wild-type mice or mice that are deficient only in a functional OAS pathway (RL -/- ) as measured by survival over 30 days. The increase in susceptibility correlated with an increase in virus titre recovered from vaginal tissue or brainstem of infected mice during acute infection. There was also an increase in CD45 + cells and CD8 + T cells residing in the central nervous system of HSV-2-infected PKR/RL -/- mice in comparison with RL -/- or wild-type control animals. In contrast, there was a reduction in the HSV-specific CD8 + T cells within the draining lymph node of the PKR/RL -/- mice. Collectively, activation of PKR, but not of OAS, contributes significantly to the local control and spread of HSV-2 following genital infection.

Herpes simplex virus type 1 ocular infection elicits a potent inflammatory response including the production of the chemokines, CXCL9 and CXCL10, in mice. Since HSV-1 nucleic acid is recognized by pattern receptors including Toll-like receptor (TLR) 9, we tested the hypothesis that TLR9 is necessary for the early augmentation of CXCL10 following HSV-1 infection. Similar to wild type controls, TLR9 deficient mice constitutively expressed CXCL10 in the cornea. Following infection or stimulation with the deoxycytidylate-phosphate-deoxyguanylate (CpG) motif, CXCL10 levels were significantly elevated in the cornea of wild type but not TLR9 or type I interferon receptor deficient mice. The reduced CXCL10 response in the cornea of TLR deficient mice was correlative with an increase in virus shedding and a reduction in neutrophil infiltration. This is the first report that shows enhanced CXCL10 expression following neurotropic viral replication requires both intact TLR 9 and type I interferon signaling pathways.

Toll-like receptors (TLRs) are innate sentinels required in clearance of bacterial and fungal infections of the cornea, but their role in viral immunity is currently ambiguous. To evaluate the role of TLRs in ocular immunity to viral infection, TLR adaptor protein deficient mice (MyD88-/- and Trif-/-) were infected with HSV-1. MyD88-/- and Trif-/- mice had similar viral loads in the cornea as those of wild type (WT) mice but substantially lower levels than type 1 interferon receptor A1 deficient (CD118-/-) mice 48-120 hours post infection (pi). A chimeric mouse model utilizing WT and CD118-/- mice was then employed and identified both a resident and bone marrow (BM)-derived immune component in viral containment 48 hours pi. The BM contribution was linked to infiltrating F4/80+ GR1+ monocytes that were severely diminished in CD118-/- mice compared to all other groups, corresponding to a loss of CCL2 production. CCL2 deficient mice were found to possess significantly more virus, consistent with a loss of F8/80+ GR1+ infiltrating monocytes in the cornea compared to WT controls. To identify the key sentinel in viral recognition of the cornea, the DNA sensor, p204, was first identified in mouse corneal epithelium by confocal microscopy and Western blot. In vivo siRNA knockdown of the sensor resulted in significant elevation of virus. Taken together, these results describe a novel TLR-independent innate immune sensor critical in the initial control of acute HSV-1 infection.

Herpes simplex virus-type 1 is among the most prevalent and successful humans pathogens. Although infection is largely uncomplicated in the immunocompetent human host, HSV-1 infection can cause blinding corneal disease, and individuals with defects in innate or adaptive immunity are susceptible to herpes simplex encephalitis. Chemokines regulate leukocyte trafficking to inflamed tissues and play a crucial role in orchestrating the immune response to HSV-1 infection. In this review we will focus on the pathways that induce chemokine expression during HSV-1 infection and the implications of chemokine signaling on control of viral replication.

The chemokine CXCL10 is among the earliest cytokines produced during nervous system inflammation induced by herpes simplex virus-1 (HSV-1) infection. Current evidence suggests CXCL10 is important in the recruitment of a wide range of leukocytes including CD4 and CD8+ T-cells, macrophages, dendritic, and NK cells. Many of these studies have suggested the function of this chemokine extends beyond simple leukocyte recruitment as CXCL10 is also expressed in primary and secondary lymphoid tissues. To elucidate the role CXCL10 has in generating an effective immune response to viral infection of the nervous system, we have analyzed leukocyte recruitment and effector function in CXCL10 deficient (−/−) mice relative to wild type C57BL6/J, CXCL9 deficient, and CXCR3 deficient mice infected with HSV-1. Our results indicate CXCL10 is critical for the recruitment of NK cells and B220+CD11c+ cells, in a CXCR3-dependent manner. Infiltration of HSV-1 specific CD8+ T-cells into the central nervous system of CXCL10 −/− mice is significantly reduced compared to wild type mice, but CXCL10 expression in lymphoid tissues does not contribute in the generation of HSV-specific CD8+ T-cells. Nervous system restricted expression of CXCL10 restored the homing of HSV-1 specific CD8+ T-cells to wild type levels and partially restored homing of natural killer and conventional dendritic (B220-CD11c+) cells.

Hematopoietic cells express TLR, raising the possibility that bacterial/viral products influence blood cell formation. We now show that common lymphoid progenitors (CLPs) express functional TLR9, and that dendritic cell production was strongly favored by TLR9 ligation with CpG ODN. TLR9 on CLPs rather than indirect cytokine mediated effects accounted for the shift in dendritic cell production, and CLPs became IL-7 unresponsive in culture. Similarly, CLPs from CpG treated mice had little ability to generate CD19+ cells and augmented competence to produce conventional (cDC), plasmacytoid (pDC) and interferon producing killer dendritic cells (IKDC) even in the absence of TNFα. Finally, lymphopoiesis in mice with active, but not latent HSV-1 infection was very similar to that seen in animals given CpG, and the changes were almost entirely mediated by TLR9. The findings bear on long standing controversies concerning the origins of dendritic cells and show how life-threatening infections may boost their production.