Chapter 10: Dynamics of Adaptive Immunity (Transcription is incomplete because recorder stopped working)
Recognition of pathogen associated molecular patterns (PAMPs) signals the cell through toll like receptors activate cells to make interferons, chemokines, cytokines, the lipid mediators such as prostaglandins and leukotrienes, and the activated cells travel from the site of infection to the local draining lymph node. For instance, if the site of infection is in the foot, and the infection is localized there, the draining lymph node is behind the knee, and while the dendritic cells travel from the foot to the lymph node they are maturing, processing the antigen and in that draining lymph node they will be able to find T-cells that are specific for that antigen, and those T-cells get activated and those T-cells will activate some B-cells along the way. The T-cells that are activated will divide and differentiate and travel back to the foot to the site of infection. This entire process takes approximately one week, and in that period, a lot has occurred in the foot in terms of innate immunity. For instance if it is a bacterial infection, macrophages have been recruited to phagocytize and eliminate those bacteria. If it is a viral infection there will be a large local interferon response, the infiltration of NK cells which will attempt to kill cells infected by the virus.
The adaptive immune response that is elicited is dependent on the type of pathogen encountered. If there is an extracellular bacteria like strep or staph the adaptive immune response will consist of antibodies that induce complement and phagocytosis. If it is an intracellular bacteria like TB or leishmania there is no way for antibody to access the bacteria, so the adaptive immune response requires activated T-cells that will enable the infected cells to eliminate the intracellular bacteria.
At the same that you are developing an adaptive immune response, the body also develops memory cells and those memory cells will be present for the rest of the persons life, and the next time that individual is exposed to that bacteria or virus they will be able to respond more rapidly, and with a larger response than during the initial infection.
Many infections are due to the breaking of skin, but infections can originate in the lungs, the GI tract, and anywhere there is infection the tissue specific macrophages and dendritic cells must respond. Once the adaptive immune response is initiated and the immune cells are activated and mature, they are released into circulation and then go to the infected tissues by moving through the capillaries that have been sensitized by the innate immune responses occurring in the tissue so the capillaries have become “sticky” and this allows the B cells and T cells to infiltrate the tissue and eliminate the infection.
If there are no innate immune responses, the host is severely compromised because any pathogen will be deadly, there must be active macrophages, dendritic cells, neutrophils, functional interferon responses; if these are absent, the initial innate response will be unable to hold the infection in check until the adaptive immune response matures. In the absence of functional T-cells, there is a sufficient innate response but the adaptive response is lost and if the innate response the insufficient, the host will die. If you have no antibody being made, but functional T-cells, the host will be able to eliminate the virus and intracellular bacteria but individuals will have repeated infections because there is no antibody present to prevent further infections.
For every action, there is an equal and opposite reaction. You expand immune responses and then shut them down, you only need to respond during the acute infection period. There are regulatory cells, T regs, including the Fox P3 expressing cells, Fox P3 is a transcription factor, and these cells can make, among other things, TGF β and sometimes IL-10 and this shuts off both the TH1 and TH2 immune response. There is another transcription factor, RORγT, and RORγT promotes the secretion of IL-17 and IL-17 is a highly pro-inflammatory molecule, it is made by T cells called TH17 cells, and these are a highly inflammatory subset of T-cells. Among many other things, TH17 cells will recruit neutrophils to the site where IL-17 is being secreted.
Whether the T cell being activated becomes a pro-inflammatory T-reg or an anti-inflammatory T-reg depends on the expression of IL-6 and IL-23. In the absence of infection, dendritic cells make high levels of TGF β and low levels of IL-6 and IL-23, when a dendritic cell making these cytokines encounters a naïve CD4 T-cell it will educate it to become a FoxP3, inhibitory T-reg. In the presence of infection, dendritic cells produce high levels of IL-6, IL-23 (these are produced in response to Toll-like receptor activation on dendritic cells) and TGF β, in the presence of high levels of these cytokines, naïve CD4 cells will be educated to become pro inflammatory RORγT expressing TH17 cells. These cells have only been described in the last 3 years, immunology doubles its database every two years. TH1 and TH2 cells were described 25 years ago, they are CD4 cells, prior to the differentiation of TH1 and TH2 immunology only recognized the difference between CD4 and CD8.
TH1 cells are also activated by a dendritic cell and antigen, but the dendritic cell that activates a naïve CD4 T cell and turns it into a TH1 cell, expresses IL-12. Dendritic cells expressing IL-12 will cause NK cells to secrete IFN-γ, and in the presence of IL-12 and IFN-γ, naïve CD4 cells will differentiate into TH1 cells. There is a positive feedback loop between cells that secrete IFN-γ (NK cells, TH1 cells and CD8 cells) and cells that secrete IL-12, IL-12 induces IFN-γ secretion and IFN-γ induces IL-12 secretion. The TH1 cell makes lots of IL-2, lots of IFN-γ, TNF and many other things. TH1 is cytolytic, all TH1 cells are capable of killing virally infected cells and cells that express class II plus their specific antigen, they can kill tumor cells, grafted cells (TH1 cells are responsible for attacking the endothelial cells on a transplanted organ).
There are other kinds of stimuli for dendritic cells that do not stimulate the dendritic cells via PAMPs but rather stimulate them in an environment where there is IL-4. Sometimes this is because an individual is atopic and they have lots of allergies and so there is a lot of IL-4 circulating in their body and the next time they encounter pathogen, their dendritic cells will be TH2 drivers; if this is the case, it can lead to more severe respiratory infections.
MyD88 is a critical signal transduction molecule within the toll like receptor pathway. If an individual has a deficiency in MyD88, then they cannot produce IFN-MyD88 is a critical signal transduction molecule within the toll like receptor pathway. If an individual has a deficiency in MyD88, then they cannot produce IFN-γ in response to viral infection or signal through any of the other toll like receptors, like TLR-2 for gram positive bacteria or TLR-4 for gram negative bacteria. Since IFN-γ and the activation of dendritic cells via this pathway, these individuals are immunocompromised and they have sluggish adaptive immune responses and they have a very difficult time dealing with viral infections. So, a normal mouse infected with toxoplasma gondii, makes lots of IL-12 and IFN-γ because they have an intact immune response and they are responding to the bacterial infection. MyD88 deficient mice can’t make IL-12 because it’s dendritic cells are not being activated through the PRRs and there is virtually no IFN-γ, as a result, the MyD88 deficient mice die, while normal mice will survive the infection.
(Page 4 slide 2) This shows some of the regulatory pathways associated with the cytokines released by these different cell types. T-regs express FoxP3 and they make TGF β which is really good at shutting down immune responses. IFN-γ is great at triggering the production of more TH1 cells, activating an increase in IL-12 released by the dendritic cells but there is a different interferon receptor present on TH2 cell than on other cells, and this instead of being an activating receptor, shuts them off. IL-10 which is made in large amounts by TH2 cells blocks antigen processing and presentation, and if you block antigen processing you will not turn on any responses and TGF β will also prevent T cell responses. On TH17 cells, the RORγT transcription factor cells, IFN-γ made by TH1 cells and IL-4 regulates the inflammation by IL-17 secreting cells. Question: What cells make IL-10? Answer: TH2 cells make IL-10 and the receptor is on the antigen presenting cell.
Experimental mouse strains have been used for many years and it has been found that BALB/C mice in general, to some antigens, are biased to make a TH2 response whereas C57/Bl6 mice in general are biased to make a TH1 response. With respect to some pathogens, having a strong TH1 response is what you need, because you have, because those CD4 TH1 cells which can kill virally infected cells and the TH1 cells which can help the CD8 cells also develop into cytolytic effector cells until the virus infection is eliminated. For other pathogens however, TH2 responses are more beneficial. For instance, if you have an intestinal parasite, you want the parasite to be expelled; some of the TH2 cytokines are good at getting the peristalsis moving and releasing the parasite. TH2 cytokines also give lots of IgE and mast cells get sensitized which is another problem. Because the BALB/C mouse is a TH2 biased mouse and because the TH1 is required to get rid of leishmania infection, the BALB/C mouse will not survive infection with leishmania. If you neutralize the IL-4 in that BALB/C mouse so that any TH1 responses are more effective because they are not countered by the development of the TH2 response, the mice survive. You can demonstrate this using KO mice for IL-4 or mice with KO for genes involved in the pathway for IL-4 production. This demonstrates how TH2 cells can result in pathology and poor survival.
Many times, we have talked about cells in circulation that need to go to a tissue site, because if they are stuck in the blood they can’t fight the tissue infection. Those cells roll along the blood vessels and normally they hold on to the selectins, and in the high endothelial venules there are a number of gene products expressed on the endothelium which grab naïve T cells and allow them to move into the lymph node. In an activated tissue, where there is an infection occurring or another immune response such as the autoimmune response in multiple sclerosis. There is a new monoclonal antibody that was developed which blocks the recognition of the CAM by VLA-4 (this prevents T cells from getting out of the blood and into the brain and causing MS flares, however this also prevents T cells from recognizing any infected activated endothelium).
There are receptors on lymphocytes, which are different depending on whether they are activated or naïve and you can distinguish this spliced variant of CD45 on active cells from resting naïve cells which have the spliced variant CD45RA, resting cells have L-selectin, which recognizes the lectin portion of CD34 and takes them out of circulation at lymph nodes, vs. the integrin recognition by CD45RO on activated cells. There are also specific cell surface molecules which go to mucosal sites and there are some which go to skin sites; it is crucial for cells to know where they’re being targeted, so that they will be taken to the right place.
So all lymphocytes, whether they are a neutrophil, a macrophage or a T-lymphocyte roll along the walls of blood vessels looking for the activated capillaries which will have certain chemokines expressed and the lymphocytes will have receptors for those chemokines, they will have receptors for the selectings and other cell surface molecules, and those interactions will bring them out to the site where an immune response is taking place. There are different chemokines, that are important to different sites but in all cases this is only one way that cells are recruited. Remember, the chemokines are not the only chemoattractants, leukotriene B4 is very important.
IL-23 is important for the differentiation of IL-17 expressing cells, with the RORγT transcription factor. IL-12 is important in the induction of TH1 cells. IL-12 and IL-23 are made by antigen presenting cells, they are heterodimeric cytokines, they share one subunit, and the shared subunit binds a shared receptor protein (these are heterodimeric receptors and there are distinct other chains of the receptors which are able to tell the cell that they have encountered IL-23 or IL-12). IL-12 is unique amongst cytokines for signaling through STAT 4 whereas IL-23 uses other STATs for signaling. The TH1 difference from TH17 is the product on the T cell that shares these receptors when it is being activated by a dendritic cell.
The ability to survive leishmania requires a strong TH1 response. TH2 responses are detrimental. If a mouse doesn’t have T-cells, if the mouse is SCID or RAG deficient, and you inject TH1 cells, those TH1 cells are able to control the infection. If you have a RAG mouse, it has no B cells or T cells, but it can process and present antigen and it has a functional innate immune system. If you have a normal mouse that cannot make IL-12 or IL-23 because it has a KO for p40 (the shared subunit in IL-12 and IL-23) it can have activated dendritic cells but those dendritic cells are incapable of providing the TH1 inducing cytokine, since these mice do not have the TH1 inducing cytokine, these mice become TH2 like and it can’t control leishmania infection.
T cell receptor requires CD4, CD8, LFA 1 interactions and positive feedback from CD28/B7 interactions. B7 is a positive signal and it turns the cell on; if you don’t have this interaction then the cell won’t be activated and when you do get activation you get effector cells. CD4 and CD8 expressing cells both require secondary interactions and cytokines from activated dendritic cell to induce differentiation.
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