The role of early life infection on the programming of CD4+ T-cells

Research Doctorate - Immunology/MicrobiologyAsthma is a chronic inflammatory disease of the airways that is characterised by activation of CD4+ T-helper 2 type (Th) cells and eosinophils. The cause of this aberrant Th2 response is unknown but lack of early life infection is thought to play a significant role. The timing of infection and the type of pathogen may be critical to programming the immune response to a protective Th1, or destructive Th2, phenotype. The immune responses to infection with Salmonella typhimurium and Mycobacterium bovis Bacille Calmette Guerin (BCG) have been identified as targets for reprogramming or preventing the development of asthma. However, the role of these infections in contributing to a Th2-Th1switch or suppression of this response remains limited. In this investigation ovalbumin (OVA) T-cell receptor (TCR) transgenic (Tg) mice in combination with these bacterial strains expressing OVA have been used to specifically track the affects of each infection as well as OVA exposure on the T-cell response and the development of allergic airways disease (AAD) in the mouse model. BCG infection as an adult and a neonate prior to OVA challenge induced significant reductions in eosinophils in broncho-alveolar lavage fluid (BALF) and lung tissue compared to sham-infected mice that received OVA challenge. However, high levels of both Th1 (interferon gamma (IFN-γ)) and Th2 (interleukin (IL)-4, IL-5, IL-13) cytokines from supernatants of cultured peri-bronchial lymph node (PBLN) cells and splenocytes were found in all groups examined. Further studies tracking the development of the immune system after BCG infection at birth without OVA exposure revealed significant decreases in lung tissue eosinophils and decreased immunoglobulin (Ig) G1, IgG2a and IgE levels from serum compared to sham-infected controls. This coincided with decreased numbers of CD4+ and CD8+ T-cells in the spleens and PBLN cells. Levels of cytokines in splenocytes and PBLN cell cultures failed to show significant trends toward either a polarised Th1 or Th2, leaving a mixed Th1/Th2 phenotype. Infection with S.typhimurium lowered eosinophil levels in BALF, and mucous secreting cell (MSC) and eosinophil number in lung tissue after challenge with 23 OVA, compared to sham-infected mice challenged with OVA. In mice infected as neonates and adults prior to OVA challenge increased levels of IFN-γ from splenocyte culture supernatants were found, compared to sham-infected OVA challenged controls. Decreased levels of IL-5 from splenocyte culture supernatants was found in neonates but not adult mice infected with S.typhimurium prior to OVA challenged compared to sham-infected OVA challenged controls. High levels of both Th1 and Th2 cytokines were present in splenocyte and PBLN culture supernatants from all groups tested, indicating a mixed Th1/Th2 phenotype rather than a profound switch to Th1 immune response. Further studies showed that infection with S.typhimurium at birth without OVA exposure causes changes to the development of the neonatal immune system resulting in decreased eosinophil numbers in BALF and lung tissue, decreased levels of serum IgG1 and IgG2a, and a shift from Th2 to a mixed Th1/Th2 cytokine profile. These changes were found in samples examined up to 9-weeks post infection. This investigation demonstrates that infection with BCG or S.typhimurium can alter the immune system resulting in attenuation of various immunological and patho-physiological features of asthma. Infection with BCG or S.typhimurium as a neonate appears to produce the most pronounced modification in the subsequent immune responses to OVA. These findings provide important insights into possible modified vaccination regimes at birth and during childhood, which may have the potential to prevent the development of asthma and allergic inflammatory disorders in adulthood