Cyclic Nucleotides and Phosphodiesterases in Monocytic Differentiation - Phosphodiesterases as Drug Targets - page 377

upregulation of PDE1B (Gantner et al. 1999 ; Giordano et al. 2003 ). Regarding
PDE4 specifically, PDE4B was found to be expressed at high levels in monocytes,
with PDE4A being the predominant PDE4 isoform in DCs (Heystek et al. 2003 ).
Elevation of cyclic nucleotides through administration of PGE 2 has been shown
to enhance the differentiation and maturation of DCs under serum-free cell culture
conditions. In lieu of fetal calf serum, a monocyte conditioned medium was used
to culture the monocytes in the presence of GM-CSF and IL-4 and the addition of
PGE 2 greatly enhanced their differentiation (Jonuleit et al. 1997 ). It was hypothe-
sized that preexposure of monocytes and DCs to cAMP-elevating agents, such as
PGE 2 , in tissues could have functional consequences for the final phenotype of the
cell and the subsequent priming of na¨ve T cells in lymph nodes.
In other studies to test the effects of elevated cAMP on DC function, monocytes
were isolated from human peripheral blood and differentiated for 6 days in the
presence of fetal calf serum, GM-CSF and IL-4 in the presence or absence of PGE 2
(Kalinski et al. 1997 ) or with the addition of the adenylyl cyclase activator
forskolin, cyclic nucleotide analogs (8-Br-cGMP and 8-Br-cAMP) or the nonselec-
tive PDE inhibitor, isobutylmethylxanthine (IBMX) (Giordano et al. 2003 ). The
phenotype of this cAMP-generated DC was characterized by analyzing its surface
marker and cytokine expression profile to determine whether the cell was directed
to a DC, or another cell type. (see Fig. 2b ) Normally during monocytic differentia-
tion to a DC, the surface markers CD14 and CD1a are reciprocally regulated with
the cells showing a downregulation of the monocytic marker CD14 and an upre-
gulation of DC marker CD1a. The reciprocal regulation of these surface markers
was prevented with increased cyclic nucleotides (Giordano et al. 2003 ). However,
cyclic nucleotides do not seem to affect the regulation of other makers such as
DC-SIGN, a DC-restricted C-type lectin involved in the early interaction between
DCs and naive T cells and also DC trafficking and internalization of Ags, or the
downregulation of the high affinity IgG-binding receptor, CD64/Fc g RI. Terminal
differentiation of myeloid cells is generally characterized by a switch from CD64/
Fc g RI to the low affinity CD32/Fc g RII. Interestingly, CD32/Fc g RII expression was
not blocked by elevated cyclic nucleotides, but was slightly enhanced and the
final phenotype was more reminiscent of the levels seen on macrophages (Giordano
et al. 2003 ). The regulation of the costimulatory molecules CD80 and CD86 was
unaffected by PGE 2 (Kalinski et al. 1997 ) but CD86 and MHC class II were
upregulated with cAMP analog treatment over the 6-day differentiation process
and the reason for this is unclear (Giordano et al. 2003 ). The authors concluded that
DC differentiation was halted at an intermediate cell stage between monocytes,
macrophages, and DCs.
This intermediate population of cells was not homogeneous; however, since
differentiation in the presence of elevated cyclic nucleotides also upregulated a
subset of cells expressing CD16 (Giordano et al. 2003 ), a surface marker upregu-
lated in some pathological conditions such as HIV and autoimmune diseases
(Grage-Griebenow et al. 2001 ). CD16 is expressed on less than 10% of circulating
monocytes and is generally expressed on the cells that are CD14 lo . After cyclic
nucleotide treatment in vitro, the percentage of CD16 + cells increased dramatically,