[Above: Failed Immunological Memory to Virulent Toxoplasma Strains.
These sequences of images show two individual mice that were vaccinated and challenged with different Toxoplasma strains expressing the luciferase gene. Bioluminescence imaging on days 1, 3 and 5 of the infection are shown (left to right) and relative parasite numbers are depicted as a heat map. The vaccinated mouse on the top row is not controlling the infection using a strain from South America, while the vaccinated mouse on the bottom is controlling infection with a commonly used laboratory strain.]
Immunological memory is the ability of our immune system to respond with greater strength and quickness upon re-encounter to the same pathogen (i.e. “secondary infection”). Vaccination elicits immunological memory responses and remains the most successful method for preventing infectious disease. The discovery, production and implementation of vaccines are one of the biggest medical breakthroughs of the last century. Smallpox has been eradicated and Polio nearly so, many other pathogens of bacterial and viral origin have been eliminated as a public health concern in many parts of the world (measles, mumps, rubella, tetanus, diphtheria, rabies, etc.). The below graphic by Leon Farrant depicts how vaccines have reduced the annual prevalence of various infectious diseases in the US (estimates are from 2008). Yet, this success has not translated to effective immune based therapies for human parasitic diseases. This fact alone is astounding, but even more so considering the global burden of human parasitic disease. One sixth of the human population currently suffers from parasitic disease leading to an estimated 96 million disability-adjusted life years (DAYLs) and 1 million deaths per year. With the exception of malaria, all major human parasitic diseases have been designated by the WHO as ‘neglected’ which often affect the poorest in third world and developing nations.
There are many reasons why vaccination for parasitic disease has been difficult. First, parasites through a myriad of virulence factors (for example see page on Toxoplasma) deflect, confuse, evade, and antagonize the immune response as they seek out essential nutrients to sustain their growth and transmission between hosts. Second, because parasites developmentally change form in the host, the breadth of antigens for protective immunity tend to be much higher, thus making the discovery of protective vaccine antigens more challenging for parasites. Many parasites use antigenic variation (Trypanosoma brucei, Plasmodium, Giardia) to evade antibody responses aimed at their elimination. Immune targeted parasitic antigens are often polymorphic adding layered complexity to vaccine design.
But perhaps most importantly, nearly all vaccines work by inducing long-lived neutralizing antibody responses by memory B cells. Yet, many pathogens and most parasites, including Toxoplasma, are not restricted by antibody neutralization alone and require sustained memory T cell responses. The big three infectious diseases of HIV-AIDS, tuberculosis and malaria require T cell mediated immunity. Inducing a long lasting memory T cell response through vaccination has been a major hurdle in vaccine development. In addition, during virulent infections T cell responses are often turned down as a programmed response to protect the host against severe inflammation and tissue damage. It is likely through this window that parasites escape elimination.
In the Jensen lab we use Toxoplasma as a model parasite to probe functional immunological memory responses to parasites. As described here, Toxoplasma strains from South American are thought to cause more severe disease in humans. Due the generation of immunological memory, previous exposure to Toxoplasma (determined by a serological test for antibodies that bind Toxoplasma surface antigens) significantly protects one from congenital toxoplasmosis (CT) during pregnancy. However, seropositive pregnant women can develop CT, suggesting Toxoplasma can evade preexisting immunological memory responses. In the one instance when the responsible strain was genotyped it was an atypical strain from South America. We recently discovered that most Toxoplasma strains from South America are virulent in chronically infected or vaccinated mice. Our results highlight that virulence is driven in many parasites by the need to prolong their survival in the mammalian host to achieve transmission. Indeed, we observed that virulence in this context was beneficial for Toxoplasma as they were able to establish chronic infection in the immune host.
[Above: Cellular immune response to initial Toxoplasma infection. This cartoon highlights some of the basic immunology underlying host resistance to primary infection. Cells of the innate and adaptive arms of the immune system are shown, as well as arrows indicating migration and communication between cells. IFNγ is central to host resistance to Toxoplasma as it activates many host mechanisms that kill the parasite.]
Initial host control of Toxoplasma infection depends on the production of the pro-inflammatory cytokine interleukin 12 (IL-12), which is produced by macrophages and dendritic cells (DCs) in response to Toll like receptor (TLR) recognition of molecular structures broadly conserved across microbial species (reviewed here). IL-12 in turn activates NK and T cells to secrete interferon γ (IFNγ). Neutrophils and γδ T cells also produce IFNγ in response to infection. IFNγ activates defense mechanisms for intracellular elimination of Toxoplasma, including the activation of interferon-regulated GTPases (IRGs), induction of reactive nitrogen intermediates, tryptophan degradation , and autophagy.
After a primary infection is cleared by the immune system, memory T cells keep the chronic infection from reactivating and causing further disease. In mouse models of chronic infection, T cells and IFNγ play pivotal roles in preventing reactivation of the dormant form. These results mirror clinical observations of HIV/AIDS patients in whom deterioration of memory T cells correlates with toxoplasmic encephalitis.
[Above: Requirements for host immunity and parasite virulence during a secondary infection. Vaccination induces memory CD8 T cell responses which are required for protection against Toxoplasma infection. Toxoplasma fights back and counters with the secreted virulence factors ROP5 and ROP18, which bind to the IFNγ -induced IRGs. Host IRGs latch on to and destroy the vacuole that Toxoplasma needs to live and grow within, leading to rapid parasite death. Virulent Toxoplasma strains have really good versions of ROP5 and ROP18 making them able to evade this particular T cell response. How then will the host control the above infection?]
Memory T cells also protect the host from secondary infections with Toxoplasma. In mouse models of vaccination, CD8 T cells are the main mediators of protection against lethal secondary challenges (here, and later here, and here), and protection is dependent on IFNγ and possibly tumor necrosis factor alpha (TNF-α) and lymphotoxin-alpha, but not perforin or iNOS. The importance of cytolysis and nitric oxide appear to be limited to chronic infection. Through a variety of genetic and genomic analysis we discovered that the Toxoplasma secreted ROP5 pseudokinases and ROP18 kinase promote immune evasion and disease in “immune” mice. Currently we are dissecting many aspects of the host pathogen battle between memory CD8 T cells and Toxoplasma. Both host and parasite genetic factors are contributing to disease outcome, which intersect the IFNγ pathway (depicted above). We are also exploring ways to help CD8 T cells fight Toxoplasma and promote immunity to South American strains.