| Zusammenfassung: | The dichotomy of T helper cell immune responses developing into the Th1 type and Th2 type, has been a very powerful means to understand the different pathways that the immune system uses to defend the host against different pathogens and tumors. Intracellular pathogens are usually controlled by Th1 type responses, while extracellular pathogens such as helminths are generally believed to be control... The dichotomy of T helper cell immune responses developing into the Th1 type and Th2 type, has been a very powerful means to understand the different pathways that the immune system uses to defend the host against different pathogens and tumors. Intracellular pathogens are usually controlled by Th1 type responses, while extracellular pathogens such as helminths are generally believed to be controlled by Th2. The majority of autoimmune diseases is thought to be due to a Th1 type over-reaction, while allergic diseases are seen as Th2 response against non-harmful environmental antigens. Over the last years, it has become clear that this dichotomy was not sufficient to explain the phenomena of suppression against autoimmunity or tumor immunity on one hand, and of immunosuppression against helminths or the mechanisms of hyposensibilisation against allergens on the other hand. In addition, one has to consider the so-called regulatory T cell responses. Regulatory T cells are characterized by low proliferation and production of IL-10 or TGF-beta. While not all subtypes of regulatory T-cells need IL-10 for their immunosuppressive action, in many situations IL-10-dependent regulatory pathways inhibit both Th1 and Th2 type immune responses.
These hypotheses would require that in some situations of autoimmunity, allergy and helminth infections, when Th2 responses are suppressed by regulatory pathways, the respective hallmark cytokines IL-4 and IL-10 will have antagonistic effects. However, direct evidence of such an antagonism has so far not been provided. In the work that led to the present thesis, antagonistic effects of IL-4 and IL-10 in two immunological model situations could be demonstrated, by using mice that have a double deficiency for both IL-4 and IL-10, in comparison to the single knockout mice.
In the first model, murine colitis that regularly develops in IL-10 knockout mice, the previous assumption that this pathology is Th1-driven would have argued that the additional knockout of IL-4 in the double knockout mice leads to a worsening of pathology. However these data clearly show that the opposite is the case: IL-4 and IL-10 double deficient mice develop colitis significantly less frequently then do IL-10 single KO mice. Using real time PCR to quantify cytokine mRNA expression from colon tissue, it could be shown that only in IL-10 knockout but not in the double deficient mice, levels of IL-12, interferon gamma, and MHCII, which are associated with the Th1 responses, were elevated at the time of onset of colitis. Additional deficiency of IL-4 in the IL-10 knockout mice prevented the expression of these genes. Immuno-staining for eosinophils revealed that this cell type is strongly increased in frequency in the colitis of IL-10 knockout mice, arguing that the pathology is not only Th1-mediated. In IL-4 and IL-10 double knockout mice, the eosinophil influx was not different from those of wildtype mice. The eosinophil influx in IL-10 knockout mice seemed to be mediated by the chemokine RANTES. Together these data show IL-4 acts antagonistically to IL-10, promoting the onset of colitis in this model.
Infection with the rodent filaria Litomosoides sigmodontis was used as the second model to demonstrate antagonistic effects of IL-4 and IL-10. In this infection, mice on the C57BL/6 background do not allow a full maturation of the infective larvae into fertile adult worms, which would produce microfilariae. Rather the worms do not develop beyond the L-4 stage/juvenile adult stage. In IL-4 knockout mice, earlier data could be confirmed that adult worms mature and release microfilariae into the bloodstream, similar to susceptible BALB/c mice. Therefore, the cytokine IL-4 is essential for the control of fertility on the C57BL/6 background. If IL-4 and IL-10 showed synergistic effects in that model, this would require that additional knockout of the IL-10 gene would lead to an even higher parasite load. However in this thesis it could be shown that the opposite is the case: additional knockout of IL-10 reverts the susceptibility, seen upon single knockout of IL-4, with no fertility of adult worms nor microfilaremia detected in the double knockout mice. Although production of interferon-gamma was increased in IL-4/10 KO mice, depletion of gamma interferon by monoclonal antibodies did not influence parasite elimination. Taken together, the results of the study add proof to the concept that has arisen for human filariasis, that IL-10 dependent responses, which are associated with patency, are antagonistic to bona fide Th2 responses, which control parasite loads.
It can be concluded from this work that situations in autoimmunity and infections diseases exists where IL-10 acts antagonistically to IL-4. This result increases our understanding of the interplay of immune responses and will hopefully be of help for the further development of immune-intervention strategies against autoimmune diseases and for vaccine design against helminth infections. |
