Biology Reference
In-Depth Information
lymphocytes ( PBLs) in patients undergoing interleukin (IL)-2 immunotherapy
led to a dramatic increase in the number of CCR5-expressing PBLs with no
accompanying increase in viral load ( Weissman et al., 2000; Zou et al., 1999).
This apparent incongruence may potentially be explained by separate obser-
vations that inhibitory CC-chemokines are a predominant product of activated
CD45RO
memory phenotypes, particularly within the interferon ( IFN-)g se-
creting subset (type-1 cells), precisely the subset that would be activated by IL-2
treatment. Multiparametric FACS analysis has the potential to clarify this
issue by combining intracellular cytokine staining with careful examination of
chemokine receptor expression in the subset involved.
It is important to realize that although in vitro experiments indicate that
threshold levels of CD4 and coreceptor are required for e½cient infection, sus-
ceptibility of primary cells in infection in vivo is dependent not only on the level
of coreceptor expression but also on the level of cognate chemokine produc-
tion. Coreceptor expression on T-helper subsets is polarized with CCR5 pre-
dominantly expressed on type-1 T-helper ( TH1) cells and CXCR4 expression
skewed toward the TH2 subset. At ®rst sight, the high level of CCR5 expression
on TH1 cells would be expected to render these cells highly susceptible to in-
fection by R5 viral strains were it not for modulating e¨ects of endogenously
produced beta-chemokines (Annunziato et al., 2000). Similarly, the extraordi-
nary amounts of ``Regulated upon Activation, Normal T cell expressed and
secreted'' secreted by megakaryocytes have been proposed as an explanation as
to why these cells are resistant to R5 but not X4 viruses despite the expression
of CD4 and both coreceptors (Chelucci et al., 1998; Majka et al., 1999; Park et
al., 1999). In addition, the constitutive secretion of SDF-1 by mucousal epithe-
lial cells may partially account for the preferential transmission of R5 viral
strains across mucosal barriers despite the ample presence of CD4
/CXCR4
lymphocytes (Agace et al., 2000). Thus, although CD4 and coreceptor expres-
sion render a cell potentially susceptible to HIV infection, the requirements for
viral entry and replication are certainly more complicated than the physical
presence of these viral receptors. External factors such as inhibitory chemokines
and post-entry factors that allow for a permissive intracellular milieu should be
taken into account, but FACS analysis of coreceptor expression is an important
component in the calculus of factors responsible for viral entry and replication.
COMPLEXITIES OF CORECEPTOR EXPRESSION REQUIRE
CAREFUL QUANTITATIVE ANALYSIS OF EXPRESSION PATTERNS
The factors a¨ecting chemokine receptor expression are legion and are beyond
the scope of this chapter (reviewed in Lee and Montaner, 1999; Murdoch and
Finn, 2000; Ward and Westwick, 1998; Ward et al., 1998). However, the com-
plexity of the network involved in regulating chemokine receptor expression on
di¨erent subsets of immune cells presents a problem uniquely suited to multi-
color FACS analysis. Unfortunately, the plethora of data regarding which fac-
