Retroviruses tend to be of great interest to the general public, because of all the media attention given to widely problematic ones, such as the Human Immunodeficiency Virus (HIV). Before effective vaccines against retroviruses like HIV can be produced, a basic understanding of how the immune system responds to them is imperative. When a retrovirus enters the human body, many are known to overcome the immune system, and possibly inhabit the host long-term. However, some animals have developed very efficient immune responses, which allow them to essentially evade or kill the virus. Scientists can study animals with this ability, like mice, to better understand the specificities of their immune systems that facilitate its advantage over the retrovirus, in hopes of relating their findings back to humans.
The first line of defense in the immune response is the innate immune system. The innate immune system is often thought of as “natural,” and provides non-specific protection against foreign invaders (1). Innate defenses are found all over the body, and include physical barriers like skin and mucus. Additionally, the innate immune system responds to pathogens in a generic way and has multiple mechanisms for killing and removing invaders. Most times, the innate defenses are successful. However, if the innate immune system is unsuccessful, it is responsible for triggering the secondary reaction of the adaptive immune system. The adaptive immune response is a specific response to a particular pathogen (foreign invader) and is usually only triggered by those entities that successfully overcome the innate immune response (1). Previous research has shown that in some strains of mice, the innate immune system senses the virus and triggers the appropriate adaptive immune response specific to the virus (2). So although the adaptive immune response is responsible for controlling retroviruses in mice, the first, critical innate virus-sensing step is still not confirmed.
Viruses are unique because they are recognized by endocytic pattern recognition receptors (PRR) on host leukocytes (white blood cells) that detect replication intermediates (nucleic acids) produced by the virus (2). However, it is unknown whether these viral sensors play a vital role in virus sensing in vivo, which is why animal models are very helpful. A recent paper by Kane et al. (2011) is therefore broadly asking whether retroviral replication intermediates are recognized by the innate immune system and further, if they are critical components for the adaptive immune response.
By using two strains of genetically retrovirus resistant mice, and two different retroviruses (mouse mammary tumor virus (MMTV) & murine leukemia virus (MuLV)), the authors hope to determine more specifically the details of retrovirus specific immunity. The first strain of mice called “B6 mice” are resistant to MuLV, but susceptible to MMTV. The second strain of mice referred to as “I/LnJ mice” are the only mice known to control for MMTV and MuLV infections through adaptive immunity (think antibodies). Kane et al. (2011) exposed each of the mice to the different viruses to determine what was happening with the innate immune system. First they wanted to see how the I/LnK mice responded to an endogenous (internal) MMTV provirus and found that the immune response was induced when virions were produced. Second, they introduced the endogenous MMTV virus to both transgenic and non-transgenic mice and found that the former responded to the virus, but the latter did not. Transgenic mice have had a provirus transgene inserted into their genome, which produces virions. Therefore, a replication-competent endogenous provirus is required for induction of an antivirus immune response to endogenous viruses. When treating I/LnJ and I/lnJ-HP (transgenic to produce virions) with exogenous MMTVs, both mice strains remained uninfected. I/LnJ mice were virus neutralizing by secretion of antibodies, while the I/LnJ-HP mice were able to activate a virus-neutralizing response produced by a replication-competent endogenous provirus. Virus resistant mice require some steps of the viral replication cycle in order to activate an antiviral immune response to endogenous retrovirus, but it remains uncertain what is necessary for the antiviral response to exogenous retroviruses.
The second phase of the experiment is built on the fact that the various responses to exogenous retroviruses are adaptive immune responses. This usually happens when one of three signals produced by the innate system initiates the adaptive response (Kane et al., 2011). All three are up regulated by external adjuvants. An adjuvant in this case is something that aids the immune system in detecting an antigen (1). The data we have so far seems to suggest that the replicating virus could substitute for the external adjuvant. The hypothesis was confirmed by treating I/LnJ mice with heat inactivated virus, in the presence or absence of an adjuvant (CFA), and finding that production of virus-specific antibodies was only found when CFA was present.
To rule out that viral entry was sufficient for activation of a virus-specific immune response, UV irradiation was used to block replication of RNA viruses, while still permitting entry. The authors treated I/LnJ mice with live or UV-irradiated MMTV or MuLV and virus specific antibodies were found in mice injected with both conditions. These results confirm that the ability to enter the host cell is sufficient to induce an antiviral response. With this knowledge at hand, the authors wanted to lastly figure out which part of the innate immune response was responsible for sensing the entry of the retroviral infection. It was hypothesized that endosomal PRR(s) would be sensing the pathogen, as replication was not deemed necessary for identification. The three possible sensors are therefore: Toll-like Receptor 3 (TLR3), TLR9, and TLR7, which detect dsRNA, DNA, and SSRNA respectively (3). B6 mice infected with MMTV or MULV had either wild type Tlr3 or knocked-out Tlr3, and in both mice, antibodies were still produced. They repeated the same with for TLR9 and found that TLR3 and TLR9 are not required for control of retroviral infections. They investigated the role of TLR7 by providing MuLV or MMTV infected mice with wild type or mutant Tlr7 and found that retrovirus-resistant mice without functional TLR7 were unable to produce virus-specific antibodies to clear the pathogen. This indicates that a TLR7-dependent mechanism is used to detect retroviruses and activate an antivirus humoral immune response. In an attempt to further confirm the role of TLR7, the authors double-checked that the up regulation of interferon regulatory factors, via TLR7, should not be given more credit. The role of TLR7 was confirmed, when IFN deficient mice, still produced antiretroviral antibodies and cleared the infection.
In conclusion, viral entry is the critical step for initiating an adaptive immune response, and TLR7 is “the retrovirus-sensing receptor.” By ultimately ruining innate immune sensing in these resistant mice, Kane et al. (2011) found that endosomal Toll-like receptor 7 (TLR7) is an innate immune receptor (PRR) that detects retroviruses by binding pathogen products in vivo (1). This study and future investigations are critical in moving forward to understanding how retroviruses work in humans.
(1) Mak, Tak & Saunders, Mary. Primer to the Immune Response. California: Elsevier, 2011. Print.
(2) Kawai, T., and Akira, S. (2010). The role of pattern-recognition receptors in innate immunity: Update on Toll-like receptors. Nat. Immunol. 11, 373-384.
(3) Kane et al. (2011). Innate immune sensing of retroviral infection via Toll-like receptor 7 occurs upon viral entry. Immunity, 35, 135-145.