Year: 2014
Type: dissertation
Abstract: The human immune system consists of the innate and the adaptive immunity that together protect the body from pathogens. To complete this task, the immune system must be able to recognize and destroy the dangerous foreign structures but also not to react to host structures or innocuous foreign structures, such as proteins of food or the commensal microbes residing in the gut. The innate immunity includes the phagocytes, such as the macrophages and neutrophils, and multiple molecular defensive systems, most importantly the complement system. The innate immunity reacts quickly to pathogens but its functions remain unchanged with repeated encounters with the intruder. The adaptive immunity is slower in its response, but it is more specific and it has memory; upon repeated exposure to a given pathogen, the adaptive immunity is activated more rapidly. Immunological tolerance, the unresponsiveness to self antigens, is a feature of the adaptive immunity. The adaptive immunity includes T and B lymphocytes and the antibodies produced by the B lymphocytes. In spite of their interdependency, the innate and adaptive immune systems have often been studied separately. This thesis focuses on their interface by investigating the role of the innate complement system in the regulation of the adaptive immunity and of the T lymphocyte function in particular. We followed the immune response and the establishment of oral tolerance in a C3 deficient mouse model, where the function of the complement system is blocked. We also studied vaccination responses and mucosal immune homeostasis in C3 deficient human subjects. The mice were immunized with ovalbumin in Complete Freund s adjuvant. In order to induce oral tolerance, some of the mice were given ovalbumin to the gastrointestinal tract prior to the immunizations. The ensuing immune response was monitored by assessing the lymphocyte fractions by flow cytometry and by stimulating splenocytes with ovalbumin and monoclonal antibodies in vitro, and measuring the proliferative response with a radioactive thymidine incorporation assay. The expression of cytokines and transcription factors in isolated cells and tissue samples was analyzed with quantitative real-time PCR. Serum antibody levels were determined by ELISA. We isolated leukocytes from peripheral blood samples collected from the patients and healthy control subjects and analyzed the lymphocyte population with flow cytometry. Serum antibodies specific for intestinal commensal microbes and the vaccine antigens tetanus toxoid and diphtheria toxoid were measured with ELISA. Serum samples were also analyzed for the presence of a set of key cytokines. The results indicate that complement plays a crucial role in the regulation of the functional differentiation of the T helper lymphocytes central to the adaptive immunity. Immunization with ovalbumin produced a weaker T cell proliferative response in the C3 deficient mouse model compared to the wild-type controls. The response of the T lymphocytes was also qualitative different, since the development of a TH1 response was particularly impaired in the absence of a functional complement system, whereas the TH2 response showed no difference between the mouse strains. This was also reflected on the B lymphocyte response: The IgG2a and IgG3 response to the immunization was reduced in the C3 deficient mice but the IgE response was normal. In addition to the general attenuation of the adaptive immunity, the C3 deficiency resulted in a disturbance of the intestinal immune tolerance in both mice and men. The administration of a foreign protein into the gastrointestinal tract of the C3 deficient mice failed to prevent the systemic immune response to the subsequent immunization with the same protein, i.e. the establishment of oral tolerance failed. The C3 deficient human subjects had more mucosally homing activated T lymphocytes in the peripheral blood and higher levels of serum IgG specific for intestinal commensal microbes. A further sign of the deficient immune tolerance in the C3 deficient human system was the lack of IgG4 response to the vaccine antigens. IgG3 antibodies specific for vaccine antigens were present at higher concentrations in the patient sera and the levels of the inflammatory cytokines IL-12 and IL-21 were also elevated. In contrast to the mouse, the profile of serum cytokines and antibody subclasses in the C3 deficient human subjects pointed at a pronounced TH1 response. The work presented in this thesis defines the complement system as a versatile regulator of the adaptive immunity and helper T lymphocytes. The normal functional differentiation of the T lymphocytes requires signals from the complement system and the establishment of immune tolerance both in the mucosal and systemic immune systems is particularly dependent on complement. The results present novel information on the interplay of the innate and adaptive immune systems and will probably affect the treatment strategies for food allergies and inflammatory bowel diseases.
Source: Työväentutkimus Vuosikirja
Author: Pirkka T. Pekkarinen
Language: English
Topic: T-cell and B-cell Immunology
Subfield: Immunology
Field: Immunology and Microbiology
Domain: Life Sciences
Open Access status: green
OpenAlex ID: W2100967363