IL-1 and its related family member
IL-18 are primarily proinflammatory
cytokines by their ability to stimulate the expression of genes associated with
inflammation and
autoimmune diseases. For
IL-1 (IL-1alpha and IL-1beta), the most salient and relevant properties are the initiation of
cyclooxygenase type 2 (COX-2), type 2
phospholipase A and
inducible nitric oxide synthase (iNOS). This accounts for the large amount of prostaglandin-E2 (
PGE2),
platelet activating factor and
nitric oxide (NO) produced by cells exposed to
IL-1 or in animals or humans injected with
IL-1. Another important member of the proinflammatory
IL-1 family is
IL-18.
IL-18 is also an important player in
autoimmune disease because of its ability to induce IFNgamma, particularly in combination with
IL-12 or
IL-15. Both
IL-1 and
IL-18 increase the expression of adhesion molecules such as
intercellular adhesion molecule-1 (ICAM-1) on mesenchymal cells and
vascular-cell adhesion molecule-1 (VCAM-1) on endothelial cells. This latter property promotes the infiltration of inflammatory and immunocompetent cells into the extravascular space.
IL-1 and
IL-18 are also an angiogenic factors by increasing the expression of
vascular endothelial growth factor;
IL-1 and
IL-18 thus play a role in pannus formation and blood vessel supply. The strongest case for the importance of
IL-1 in disease processes come from the administration of the
IL-1 receptor antagonist, also a member of the
IL-1 family and
IL-18 binding protein (IL-18BP), a constitutively expressed and secreted
protein that binds and neutralizes
IL-18. Data from the human genome project have revealed other members of the
IL-1 family. However, these appear to be antagonists rather than agonists.
IL-1 also acts as an adjuvant during antibody production and stimulates bone marrow stem cells for differentiation in the myeloid series.
IL-1 is distinct from
tumor necrosis factor (TNF);
IL-1 and
TNFalpha share several
biological properties but the salient difference is that
TNF receptor signaling induces programmed cell death whereas
IL-1 receptor signaling does not. In fact,
IL-1 is a hematopoietic
growth factor and
IL-1 was administered to humans to reduce the nadir of white blood cells and platelets in patients during
bone-marrow transplantation. This property, of
IL-1 is not observed in the responses to
TNFalpha. Furthermore, in animal models of destructive
rheumatoid arthritis,
IL-1 is necessary but
TNFalpha is not.