Chapter 13
Innate and Adaptive Immunity
279
Chemokines are named according to structure, followed
by “L” and the number of their gene (
e.g.,
CCL1, CXCL1).
Likewise, chemokine receptors are named according to the
structure, followed by an “R” and a number (
e.g.,
CCR1,
CXCR1). Six receptors for CXC (CXCRs) and 10 for CC
(CCRs) chemokines have been characterized in terms of their
structure and function.
2
Chemokines communicate with their
target cells by activating G-protein–coupled receptors that are
pertussis toxin sensitive and as a result are capable of activat-
ing different populations of leukocytes, thereby controlling
the migration of immune cells to their sites of action based
upon the needs of the situation.
2
Most receptors recognize
more than one chemokine, and most chemokines recognize
more than one receptor. Binding of a chemokine to a receptor
can result in inhibition or activation with the same chemo-
kine acting as an activator at one type of receptor and as an
inhibitor at another. Chemokines are implicated in a num-
ber of acute and chronic diseases, including atherosclerosis,
rheumatoid arthritis, inflammatory bowel disease (Crohn
disease, ulcerative colitis), allergic asthma and chronic bron-
chitis, multiple sclerosis, systemic lupus erythematosus, and
HIV infection. They also play a role in the body’s immune
response against cancer cells through the up-regulation of
CCL21 and other chemokines by activated T cells and other
tumor-derived proteins.
7,8
Colony-Stimulating Factors
Colony-stimulating factors (CSFs) encompass a subset of
cytokines that participate in hematopoiesis by stimulating
bone marrow pluripotent stem and progenitor or precursor
cells to produce large numbers of mature platelets, erythro-
cytes, lymphocytes, neutrophils, monocytes, eosinophils,
basophils, and dendritic cells (DCs). The CSFs were named
according to the type of target cell on which they act (see
Table 13.2). Macrophages, endothelial cells, and fibroblasts
produce granulocyte colony-stimulating factor (G-CSF)
during times of stress and inflammation where it promotes
growth and maturation of neutrophils. Granulocyte/monocyte
colony-stimulating factor (GM-CSF) acts on the granulocyte–
monocyte progenitor cells to produce monocytes, neutrophils,
and DCs, and monocyte colony-stimulating factor (M-CSF)
stimulates the mononuclear phagocyte progenitor. While CSF
is necessary for normal blood cell production and maturation,
excess CSF production has been implicated in several disease
processes and the development of corticosteroid-resistant
chronic obstructive pulmonary disease (COPD).
9
Impaired
macrophage function and subsequent impairment of G-CSF
activity have been associated with the development of neu-
trophilia in animal studies.
10
In clinical practice, recombinant
CSF is being used to increase the success rates of bone mar-
row transplantations. The availability of recombinant CSFs
and cytokines offers the possibility of several clinical thera-
pies where stimulation or inhibition of the immune response
or cell production is desirable.
formed molecules but rather are synthesized through tran-
scription as a result of cellular activation. The actions of
cytokines are often pleiotropic, meaning that they have the
ability to allow a single cytokine to act on different cell types.
For example, IL-17 is produced by the T-helper 17 (T
17
H)
cells and acts on several cell types including leukocytes, epi-
thelial cells, mesothelial cells, vascular endothelial cells, and
fibroblasts. As a result, T
17
H cells play a critical role in host
defense against pathogens that infiltrate the mucosal barrier.
5
Although pleiotropism allows cytokines to mediate diverse
effects, it greatly limits their use for therapeutic purposes
because of numerous unwanted side effects. Redundancy
refers to the ability of different cytokines to stimulate the
same or overlapping biologic functions. Because of this
redundancy, antagonists against a single cytokine may not
have functional consequences because other cytokines may
compensate.
In addition to being pleiotropic and redundant, cyto-
kines can have broad activity. Therefore, several different
cell types are capable of producing a single cytokine. For
example, IL-1 is a proinflammatory cytokine that is primarily
produced by macrophages but can be produced by virtually
all leukocytes, endothelial cells, and fibroblasts. Cytokines
also function to initiate cascade functions with one cytokine
influencing the synthesis and actions of other cytokines.
Often the second and third cytokines may mediate the bio-
logic effects of the first cytokine. These effects may be local-
ized, acting on a single cell or group of cells in the area sur-
rounding the effector cell, or the effects can be systemic with
the cytokines secreted into the bloodstream and transported
to their site of action. TNF-
α
is an example of a cytokine with
wide-reaching systemic effects. Cytokines may also serve as
antagonists to inhibit the action of another cytokine and as a
result act as anti-inflammatory cytokines. IL-110 is an anti-
inflammatory cytokine to down-regulate the inflammatory
and adaptive immune responses.
Chemokines
Chemokines are small protein molecules (70 to 130 amino
acids) that are involved in immune and inflammatory cellu-
lar responses and function to control the migration of leuko-
cytes to their primary site of action in the immune response.
6
There are four distinct classes of chemokines (C, CC, CXC,
and CX3C), which are named for the number and location of
cysteine residues on the terminal amino acid of the protein.
2
Currently, 48 distinct chemokine molecules have been iden-
tified within the four different classes. The vast majority of
these are classified as either CC or CXC chemokines. The CC
chemokines have the first two cysteine molecules adjacent to
each other, while these molecules are separated by an amino
acid in the CXC chemokines. The CC chemokines attract
monocytes, lymphocytes, and eosinophils to sites of chronic
inflammation. The CXC chemokines attract neutrophils to
sites of acute inflammation.
