The Role of Macrophages in Regulating Inflammation by Oxidative Burst

The production of reactive oxygen species (ROS) through a process called oxidative
burst is an essential defence mechanism against pathogens. In phagocytes, such as
neutrophils and macrophages, the NADPH oxidase 2 (NOX2) complex is the main
source of ROS. Genetic alterations in any of the components of the NOX2 complex that
impair the ROS production are at the origin of a condition called chronic
granulomatous disease (CGD), characterized by recurrent life threatening bacterial and
fungal infections. Recently, natural occurring mutations in Ncf1, a regulatory
component of the NOX2 complex, were described to compromise the protein function
and to increase arthritis severity in rats and mice. Macrophages are phagocytes that
express NCF1 and are able to kill pathogens. At the same time, they are antigenpresenting
cells known to play an important role in arthritis. We therefore hypothesized
that expression of NCF1 in macrophages would have an impact on the immune
response during arthritis and bacterial infections. The aim of the studies presented in
this thesis is to evaluate the influence of NCF1 expressed by macrophages on
development of arthritis and resolution of bacterial infections.
Using transgenic mouse models we could describe a role for macrophages in both
priming and activation of arthritogenic T cells. In a first transgenic mouse, expression
of functional NCF1 restricted to macrophages reduced arthritis severity, priming of Th1
T cells and T cell proliferation, therefore limiting the T cell-dependent autoimmune
outbreak. In a second transgenic mouse strain, where macrophages were the only cells
expressing the arthritis-prone MHC class II Aq molecule, macrophages could prime
arthritogenic T cells and mediate arthritis development, but only in NCF1 deficient
setting. We could conclude that ROS production by macrophages is important in
determining the activation state of T cells and in regulating the severity of arthritis.
As in the human CGD situation, mice carrying the Ncf1 mutation were more
susceptible to spontaneous and induced bacterial infections. Using the transgenic
mouse where macrophages expressed the functional NCF1, we observed that
macrophage-derived ROS effectively protected mice from bacterial infections, a
function believed to be executed mainly by neutrophils.
Finally, we tested a new model of arthritis where the disease was induced with a
peptide of a glycolytic enzyme. We found that the symptoms and pathogenesis of the
disease resembled the one of the most common arthritis model, collagen-induced
arthritis, which is induced with the full collagen protein. Both diseases are dependent
on an intact adaptive immune system and their severity is influenced by Ncf1. We were
also able to identify one of the important residues causing the peptide’s arthritogenicity.
In summary, our data highlight the crucial role of NCF1, and consequently of NOX2
complex, in regulating both innate and adaptive immune responses. NCF1-dependent
ROS in macrophages was important during both phagocytosis and antigen presentation,
resulting in clearance of bacterial infection and suppression of chronic inflammation.
These findings will facilitate further investigations of the molecular pathways through
which ROS influence arthritis pathogenesis and hopefully lead to identification of new
therapeutic targets