The innate immune system, that part of the immune system that responds to non-specific signs of pathogen infection, faces a quandary when mounting a response against a pathogen. On one hand, it must limit the replication of the pathogen to prevent spread throughout the body. On the other, it must facilitate the development of a specific adaptive immune response, which can clear the pathogen from the body and lead to immunologic memory. It might seem like both of these are reasonable tasks, but the quandary lies in the fact that the adaptive immune system must be activated by a certain amount of antigen (proteins derived from the pathogen that can be effectively targeted by the immune system) in order to elicit an effective response. Low levels of antigen lead to weak responses, whereas high amounts of antigen induce strong responses. Thus, if the innate immune system does the first part of its job too well and largely prevents virus replication, there will not be enough viral antigen present to stimulate a robust adaptive response. So how does the immune system get around this predicament? A new paper in Nature Immunology by Nadine Honke and her colleagues sheds some light on this question (1).
The major weapons in the innate immune system’s arsenal are small, secreted proteins called interferons. When cells in the body detect a pathogen, they produce interferons, which alert the surrounding cells to the impending danger. These “warning flags” bind to interferon receptors found on the surface of all cells, and induce the expression of a large number of genes, termed interferon stimulated genes (ISGs), which can combat the pathogen through various means. Interferons also initiate mechanisms that facilitate the adaptive immune response, by increasing the presentation of specific antigens to the T cells and B cells that comprise the adaptive immune system. To effectively stimulate T cells, however, requires a sufficient dose of antigen: prior studies have indicated that over 20,000 copies of the antigen are required for an antigen-presenting cell to induce a robust T cell response (2). So, how can the innate immune system effectively prevent the spread of a pathogen while still providing enough “grist” to be presented to naïve T cells? Honke et al began to answer this by examining a particular subset of innate immune cells, metallophilic macrophages.
Macrophages are phagocytic cells, which means that they engulf and digest pathogens and cellular debris that they come into contact with. They play a role in both innate immune processes, through their non-specific destruction of pathogens, and adaptive immune processes, through their ability to activate T cells and B cells. Metallophilic macrophages, a subset of these cells also known as CD169+ macrophages, reside in the lymph nodes and spleen, and filter the blood for antigens. This makes them a good candidate for facilitating adaptive immune activation. Indeed, when the authors depleted these macrophages from mice and then challenged them with vesicular stomatitis virus (VSV), the virus remained in the blood stream for longer periods than observed in normal mice, suggesting that these cells efficiently filter the virus from the blood. The macrophage-depleted animals also succumbed to the infection, unlike normal mice, indicating an important role for macrophages in protecting the mice from lethal infection. The authors examined virus replication in wild-type mice and mice that genetically lacked the interferon receptor, and found that, as expected, in the absence of interferon signaling, the virus replicated to high levels throughout the body. In the wild-type mice, however, virus replication was only observed in the spleen, the location of the CD169+ macrophages. When the authors looked for the location of the virus antigen in the spleen, they found replicating virus only in the CD169+ population (see figure). These results suggest that this population of cells fosters virus replication, even in the presence of effective interferon-mediated inhibition of infection in neighboring cells and tissues.
So how can CD169+ macrophages allow virus replication, despite the antiviral effects of interferon? The answer lies in the cellular protein Usp18. This protein is known to suppress interferon signaling (3), and Honke et al showed that it was expressed at high levels in the CD169+ macrophages, but not in other macrophage subsets. Mice that lacked Usp18 exhibited no VSV antigen in the CD169+ population, and much lower titers of VSV in the spleen. Thus, the metallophilic macrophages inure themselves to the antiviral effects of interferon by expressing Usp18, in order to allow robust virus replication!
The next step was to link this phenomenon to the development of the adaptive immune response. Honke et al found that the production of neutralizing antibodies by B cells was significantly diminished following depletion of CD169+ macrophages, and both T cell and B cell responses were attenuated in mice lacking Usp18. No differences were observed between wild-type and Usp18-deficient mice when challenged with inactivated VSV, indicating that replication-competent virus was necessary for this effect. These defects in adaptive immune responses had consequences on the outcome of the infection, as VSV was able to spread and cause a lethal central nervous system infection in the Usp18-deficient animals.
To summarize, interferons function to limit virus replication in most cells in the body. CD169+ macrophages express Usp18 to suppress this response, allowing for compartmentalized virus replication as a means of facilitating a large enough antigen load to adequately prime the adaptive immune response. It certainly seems counterintuitive to allow a virus to replicate at all when the means of suppressing replication are readily available. Nevertheless, this trade-off facilitates an even more effective adaptive immune response, and the subsequent generation of immunologic memory. One outstanding question is whether this subversion of the interferon response is complete, or just targeted toward a handful of ISGs. Many ISGs are not directly anti-viral, but function to increase antigen presentation, and so might not be beneficial to inhibit even in the CD169+ macrophages. The subset of ISGs inhibited by Usp18 expression in these cells remains to be elucidated.
Overall, this manuscript has wide-ranging implications, especially for the development of vaccines. It suggests that “live-attenuated” vaccines, which feature replication-competent pathogens that have been neutered of their disease-causing features, may be a far more efficient means of eliciting a protective immune response than “inactivated” vaccines, which are replication-incompetent. While this is certainly not a novel idea, this paper reveals part of the mechanism behind this phenomenon and perhaps might allow researchers to more efficiently target vaccines to the cell types, such as CD169+ macrophages, that can elicit the most robust immune response.
(1) Honke, N., et al. Enforced viral replication activates adaptive immunity and is essential for the control of a cytopathic virus. Nat. Immunol. 13, 51-57 (2012).
(2) Henrickson, S.E. et al. T cell sensing of antigen dose governs interactive behavior with dendritic cells and sets a threshold for T cell activation. Nat. Immunol. 9, 282–291 (2008).
(3) Ritchie, K.J. et al. Role of ISG15 protease UBP43 (USP18) in innate immunity to viral infection. Nat. Med. 10, 1374–1378 (2004).