Biology Reference
In-Depth Information
4.2 The PDE4-Regulated Inflammatory Cells: Neutrophils,
Eosinophils, Basophils, Monocytes
Human neutrophils represent proinflammatory cells, which can be isolated easily
and routinely from human blood. After comparing activators such as opsonised
zymosan or C5a it appeared that fMLP was the most reliable activator for degranu-
lation, secretion and release of reactive oxygen species (ROS). Chromatography
of neutrophil extracts (which at that time had not been carefully separated from
adherent platelets) showed one prominent peak, which appeared to be highly
sensitive to rolipram and Ro 20-1724 (Schudt et al.
1991a
). Quantitation of ROS
release from neutrophils with the cytochrome C method was introduced by Lad
et al. (
1985
), who demonstrated the reduction of the oxidative burst by prostaglan-
dins, isoproterenol and IBMX. Analysis of the relationship and cellular inhibition
by this method revealed correlations between biochemical characteristics of PDE4
inhibitors (IC50 of PDE4) and the pharmacological effects of these inhibitors
on ROS release in neutrophils (EC50 of ROS) (Schudt et al.
1991c
) and eosinophils
(Barnette et al.
1995b
). These published correlations, however, were no longer true
when from the beginning of the 1990s, the detection method for ROS release was
changed to the much more sensitive chemoluminescence measurements (Nielson
et al.
1990
; Schudt et al.
1991b
).
Eosinophils were known to be crucially important in asthma, since asthma was
denominated as “eosinophilic bronchitis” (Kay
1987
). Eosinophils were usually
isolated from guinea pigs, which frequently exhibited an endogeneous eosinophilia.
Thus, these proinflammatory cells became available for testing new inhibitors and
were investigated in the group of PJ Barnes (Dent et al.
1990
,
1991
). It emerged that
(1) the oxidative burst of eosinophils was sensitive to PDE4 inhibitors, and since
PDE4 was the only cyclic nucleotide-degrading enzyme in these cells, other
secretory parameters such as (2) LTC4 and (3) eosinophilic cationic proteins
(ECPs) were also determined to be affected by PDE4 inhibition (Hatzelmann
et al.
1995
; Tenor et al.
1996
).
Basophils and mast cells are responsible for histamine and LTC4 release (
s
low-
r
eacting-
s
ubstance of
a
naphylaxis, SRS-A at that time). Strips of human lung tissue
can be stimulated to contract by IgE, and it was already observed in 1972 in the
groups of Liechtenstein and Austen that cAMP analogs, PGE2 or theophylline
could inhibit this release which resembles the “relieving” mechanisms counter-
acting the primary allergic attack (Liechtenstein and Margolis
1968
; Kaliner et al.
1972
). Later, in basophils from atopic patients PDE inhibitors reduced mediator
release, enhanced cAMP concentration and the cAMP/PKA involvement in the
secretory mechanisms was demonstrated (Marone et al.
1987
). The reduction
of antigen-induced histamine release by PDE4 inhibition was described by
Kleine-Tebbe in atopic basophils (Kleine-Tebbe et al.
1992
). Mast cells would
have been the desired model for allergic inflammation, but due to their complicated
isolation the model of lung strips from atopic animals was used instead in many
laboratories (see Sect.
3.3.6
).
