Local advantage: skin DCs prime, skin memory T cells protect

Abstract

How the immune system responds to local infection and establish protective immunity within the susceptible tissue remains unclear. Two new studies show local tissue-resident DCs prime CTL responses and memory CTLs remain within the tissue to provide antiviral immunity.

Cytotoxic T lymphocytes (CTLs) are key effector cells that provide protection from viral infection. CD8⁺ T cells bearing T cell receptors specific for a given viral antigen are primed within the secondary lymphoid organs by dendritic cells (DCs). Exactly which subset of DCs prime CTLs in response to viral infection has been an area of intense debate. Many pathogens enter the host via a specific niche, most commonly through the mucosal surfaces, where they establish a local acute and chronic infection. Herpesvirus family members represent a good example of such a pathogen. In particular, herpes simplex virus type 1 (HSV-1) enters the human host through the oral mucosa and establishes latency in the trigeminal ganglia. Reactivation of the latent HSV-1 in the ganglia leads to anterograde transport of virus back to the skin, causing “cold sore” lesions around the mouth. CTLs play an important role in controlling both the reactivation of HSV-1¹ and in limiting viral replication within the peripheral site of replication². Given the restricted nature of HSV-1 replication within the epithelial cells and latency within the innervating ganglia, what cells prime CD8⁺ T cells in the draining lymph node and how protection is afforded by memory T cells are important unresolved questions. In this issue of Nature Immunology, two papers reveal that local tissue-resident cells do both – Heath and colleagues show that CTL priming is primarily carried out by CD103⁺ Langerin⁺ dermal DCs (DDCs)³, while Carbone and colleagues report that memory CTLs in the skin remain within the tissue and maximize protective immunity against subsequent challenge with HSV-1 (Ref 4).

Within the skin, three DC subsets are present – the Langerhans cells in the epidermis, and two subsets of dermal DCs (DDCs), consisting of Langerin⁻ DDCs and the newly described Langerin⁺ CD103⁺ DDCs^5–7 (Fig. 1). Traditionally, it was thought that local tissue-resident DCs phagocytose microbial antigens and migrate to the draining lymph node to prime naïve T cells. However, this paradigm has been challenged by multiple studies in which lymph node-resident DCs were shown to be the only APCs that present antigens to T cells. With respect to the DCs involved in CTL priming, the importance of the lymph node-resident CD8α⁺ DCs has become well accepted⁸. The CD8α⁺ DCs are the predominant APCs for CTLs that are generated following infection by HSV-1, influenza virus, vaccinia virus, lymphocytic choriomeningitis virus and Listeria monocytogenes. CD8α⁺ DCs could exclusively prime CTL responses whether HSV-1 was injected by needle into the footpad, via the intravenous route or by dermal abrasion⁹ (Fig. 1a). In the current study, Bedoui et al. made an intriguing observation that reactivating HSV-1 causes a second phase of infection of the entire skin dermatome innervated by the infected ganglia (Fig. 1b), resulting in a second wave of antigen presentation in the lymph nodes draining the new site of viral replication. Reactivating HSV-1 replicated within the epithelium, importantly in the absence of any artificial manipulation of the skin, allowing the authors to study the course of natural infection. Taking advantage of this system, Heath and colleagues assessed DC subsets for their ability to stimulate CTLs. Strikingly, they found that while cross presentation of viral antigen during primary infection following scarification was mediated by the lymph node resident CD8α⁺ DCs^3,9, antigen presentation following natural infection with HSV-1 in the skin during recrudescence was handled almost exclusively by the CD103⁺ DDCs (Fig. 1b). These results are consistent with the fact that DDCs are the predominant APCs following natural infection of vaginal mucosa with HSV-2 (Ref 10), and also are supported by a recent study that showed differential participation of migrant versus lymph node resident DCs in CTL priming following natural mucosal infection versus. skin abrasion with HSV-1, respectively¹¹.

Figure 1.

Induction and execution of immune responses against HSV-1 are coordinated by local DCs and memory T cells. (a) Primary infection by HSV-1 is initiated by mechanical scarification of the superficial layer of the epidermis, allowing the virus to enter and replicate locally. HSV-1 infects the innervating ganglion by retrograde transport from the nerve endings in the skin, and establishes latency. Virus introduced via this route is taken up by local skin-resident DCs, which upon migration, present antigens to CD4⁺ T cells. However, cross-priming of CD8⁺ T cells is uniquely carried out by the lymph node-resident CD8α⁺ DCs. (b) Reactivation of the latent virus in the ganglion results in the anterograde migration of infectious virions to the skin and infection of the epithelial cells throughout the dermatome innervated by the ganglion. Following this naturally route of reinfection, the viral antigens are cross-presented to CD8⁺ T cells by CD103⁺ Langerin⁺ DDCs and not the CD8α⁺ DCs. (c) CTL induced by DCs differentiate into three kinds of memory cells, T_CM, T_EM and T_RM. Only the T_RM take up residency in the skin (at the previous site of virus infection) and near the latently infected ganglion. Upon tertiary infection by HSV-1, T_RM provide bulk of the protection even though T_EM can also be recruited to the site from systemic circulation.

Several intriguing questions arise from this study. First, why are different DCs involved in CTL priming during the primary and secondary infection? Is it possible that scarification allows HSV-1 to be carried via the lymph, circumventing the requirement for migrant DC presentation? This is unlikely since migrant DCs are still required for the CD8α⁺ DCs to prime CTL immunity following scarification⁹, indicating that even if direct entry of the virus into the lymph node does occur, it is insufficient for priming by CD8α⁺ DCs. Because scarification causes significant tissue damage, it is conceivable that the CD103⁺ DDC functions may be suppressed by tissue-derived factors, rendering them incapable of priming CD8⁺ T cells. Langerin⁺ DDCs were shown to be responsible for cross-presenting epidermally expressed self-antigen³, and are required for contact hypersensitivity responses⁷, suggesting that these cells are capable of presenting a diverse set of antigens. In contrast, Langerin⁻ DDCs are the predominant APC for CD4⁺ T cells following scarification-induced HSV-1 infection and following natural reactivation (Fig. 1a, b). Thus, it will also be important to understand cellular and molecular mechanisms by which CD103⁺ DDCs versus CD103⁻ DDCs prime CD8⁺ T and CD4⁺ T cells. Second, which wave of T cell priming results in the establishment of protective immunity? Are the effector and memory CTL induced by the first and second wave of HSV-1 infection quantitatively and qualitatively similar? The answers to these questions will not only be important for development of vaccines against HSV-1, but will also provide key clues to the findings reported by Carbone et al⁴.

Once primed by DCs, the virus-specific CD8⁺ T cells differentiate into at least two types of memory cells. The central memory T cells (T_CM) home to lymphoid organs and have limited effector functions, whereas effector memory T cells (T_EM) home to peripheral tissues and rapidly secrete cytokines¹². In the current study⁴, Carbone and colleagues propose the existence of another type of memory T cells, tissue resident memory cells, or T_RM. The authors carry out an elegant set of transplantation studies to demonstrate that T_RM reside both close to the latently infected ganglia and in the skin close to the primary site of HSV-1 infection (Fig. 1c). Unlike T_CM and T_EM, T_RM do not readily enter circulation once they establish residency in a given tissue, and can proliferate locally upon secondary viral challenge. Strikingly, T_RM enter and remain within the tissue even in the absence of virus and presumably viral antigens. This was demonstrated by the fact that scarification alone induced recruitment of T_RM to damaged tissue, and that skin containing T_RM, when transplanted into naïve recipient, retained T_RM for over 3 weeks even though it was separated from the latently infected ganglia. Most importantly, when secondary HSV-1 challenge was applied to previously infected flank (containing T_RM) or to the opposite flank (only able to recruit T_EM), a hundred-fold less virus was found in the T_RM-containing tissue compared to site in which only the T_EM were newly recruited. However, T_EM still provided some protection over unimmunized control by reducing viral load by a hundred fold. These data show that complete protection against a viral challenge requires not only systemic CTL memory but also T_RM must be mobilized to the site of potential viral encounter prior to infection.

T_RM could represent a distinct lineage of memory cells that arise from the effector CTL pool, or they may differentiate from T_EM once they arrive in the infected/damaged tissue. T_RM express high amounts of VLA-1 and CD69 but are CD62L^lo and CD122^lo. However, this expression pattern is also shared by T_EM. If T_RM are a distinct lineage of memory cells, what factors influence their development? On the other hand, if T_EM do differentiate into T_RM, it implies that the chemokines that recruit T_EM will be responsible for the eventual existence of T_RM in a given tissue. The cues that are responsible for conversion of T_EM to T_RM and how long it takes will be important areas of research. Another question that needs answering is how, given viral antigen is not required, T_RM are retained in a given tissue. This is particularly relevant for autoimmune diseases where T_RM may be causing chronic tissue destruction.

These questions aside, the findings reported by Gebhardt et al.⁴ have important implications in vaccine development. Most pathogens gain a foothold in the host through specific portals of entry. For example, HIV-1 enters via the genital and rectal mucosa while MTB gains access via the respiratory mucosa. As such, providing effective protection will require mobilization of T_RM to the appropriate mucosal surfaces.Once again, the mechanism of T_RM recruitment and retention in a given tissue needs investigating. One unconventional proposal would be to “scratch and save (life)” – by creating minor scarification to recruit T_RM to the potential site of pathogen encounter after a conventional parenteral immunization. Of course, not all surfaces are conducive to this type of manipulation. A more universal and powerful approach would be to immunize at the potential site of pathogen encounter. Development of safe mucosal vaccines capable of establishing T_RM at the site of pathogen entry may be the answer to preventing transmission of deadly virus infections in humans.

In conclusion, the studies by the groups of Heath³ and Carbone⁴ provide important insights on the priming and execution of antiviral immunity to a local viral infection, and open new avenue of investigation and, possibly, therapeutic approaches. With the development of a new in vivo approach to temporally and selectively deplete the Langerin⁺ DDCs⁷, future studies are expected to reveal the role of these cells in immune responses to a variety of antigens. In addition, unveiling of the biology of T_RM will provide clues for tissue-specific memory generation that is needed for vaccine development and for immune intervention for autoimmune diseases.