Innate and adaptive immune responses in pulmonary sarcoidosis

Abstract

Sarcoidosis is an inflammatory disease characterized by formation of granulomas in various organs and tissues. Although it is a systemic disorder, the lungs are commonly affected, and an infiltration of T cells, in particular CD4pos T helper 1 (Th1) cells, is seen in the lower airways. HLA-DRB1*0301pos (HLA-DR3pos) patients commonly present with acute disease onset, typically with Löfgren´s syndrome, accumulation of lung CD4pos T cells expressing the T cell receptor (TCR) gene segment AV2S3 (AV2S3pos T cells) and good prognosis. In contrast, HLA-DR3neg patients show insidious disease onset and are at risk of developing lung fibrosis. The aetiology of sarcoidosis is still unknown; however, several studies indicate an infectious cause, suggesting a role for innate immune receptors, including toll-like receptors (TLRs) and nod-like receptors (NODs). Recently, the mycobacterial protein mKatG was identified in sarcoidosis tissues but not in healthy subjects, and T cell responses have been reported to mKatG. However, their specificity and cytokine profile, as well as the phenotype of lung-accumulated T cells in general, has not been that well described.

The aim of this thesis was to examine differences between patients and healthy subjects, as well as between distinct patient subgroups, regarding innate and adaptive immune responses in the affected organ, i.e. the lung, and in the blood.

Sarcoidosis patients had, as expected, an enhanced Th1 mediated immune response in their lungs, as compared to healthy subjects. HLA-DR3pos patients had a lower IFNγ and TNF gene expression in BAL cells, and tendencies to lower IL-2 and TNF protein levels in BAL fluid, than HLA-DR3neg patients, suggesting that a less pronounced Th1 immune response in HLA-DR3pos patients may be related to their good prognosis.

Blood monocytes of patients had higher baseline TLR2 and TLR4 expression, as compared to healthy subjects, which could have consequences for host-pathogen interaction. Stimulation with TLR2 and NOD2 ligands in combination resulted in a much higher production in blood cells from patients of IL-1β and TNF, which could promote the initiation of inflammatory responses. In addition, combined TLR2 and NOD2 stimulation gave rise to a synergistic induction of IL-1β in patients, whereas IL-10 was synergistically induced in healthy subjects. Total BAL CD4pos T cells of patients with Löfgren´s syndrome (including HLA-DR3pos patients) had a more pronounced multifunctional cytokine profile, i.e simultaneous production of IFNγ and TNF, after stimulation with mKatG, whereas total BAL CD4pos T cells of non-Löfgren patients exhibited a predominantly single-functional cytokine profile, i.e. production of IFNγ alone. This indicates that the quality of T cell responses may be of critical importance for the clinical presentation and disease outcome. BAL and blood TCR AV2S3pos T cells of HLA-DR3pos patients produced more IFNγ in response to mKatG, as compared to TCR AV2S3neg T cells, whereas the opposite was seen in BAL after stimulation with PPD. This, together with our previous knowledge of HLA-DR3pos patients having a good prognosis, implies that the role of TCR AV2S3pos T cells is to eliminate an offending antigen.

HLA-DR3pos patients had a reduced frequency of FoxP3pos regulatory T cells among their BAL CD27pos and CD27neg memory T cell subsets, as compared to HLA-DR3neg patients. HLA-DR3pos patients had a higher frequency of naïve CD25negCD27pos BAL T cells, as compared to HLA-DR3neg patients, implying that the total T cells are less activated in HLA-DR3pos patients, which is in line with the finding of a less pronounced Th1 response in the lungs of these patients. BAL TCR AV2S3pos T cells, that exhibited a CD27pos memory phenotype, were much less positive for the regulatory marker FoxP3, as compared to the TCR AV2S3neg T cells, further supporting that TCR AV2S3pos T cells are effector cells rather than regulatory cells. BAL and blood TCR AV2S3pos T cells were more activated and differentiated than TCR AV2S3neg T cells, indicating their encountering with a postulated sarcoidosis antigen in vivo.

Taken together, the findings presented in this thesis reveal that patients with good prognosis in their lungs exhibit an efficient multifunctional cytokine profile of mKatG-specific T cells, a reduced number of FoxP3-expressing memory T cells, an increased number of naïve T cells, and highly activated TCR AV2S3pos effector T cells, which we suggest all contribute to an efficient immune response, focused towards elimination of the sarcoidosis antigen(s), followed by spontaneous recovery.