Biology Reference
In-Depth Information
ceptor expression, and hence viral infectivity. In this chapter, we will review
how the expression patterns of CCR5 and CXCR4 impact HIV pathogenesis
and point out examples of how multiparametric FACS analysis of coreceptor
expression has provided important information on HIV-receptor interactions.
In addition, investigations into the e½ciency of coreceptor usage, which may
impact the pathogenicity of the viral isolate in question, also demands the
ability to quantify the number of coreceptor molecules on the cell surface. We
will review the use of quantitative FACS as a way to put absolute numbers on
the levels of coreceptor expression and argue that future studies involving
coreceptor expression levels adopt this methodology so that results from vary-
ing groups can be more readily compared.
EXPRESSION PATTERN AND LEVELS OF CORECEPTOR HAVE
IMPLICATIONS FOR HIV PATHOGENESIS
Early in the HIV/AIDS epidemic, the level of circulating CD4-positive T cells
provided the most easily quanti®able parameter that could be used to measure
disease progression. Although still providing important information on immune
status, measurements of virus load (the number of viral RNA molecules per
milliliter of blood) are now the best predictor of disease course. Similarly, it is
now apparent that simple measurement of CD4 levels per se is overly simplistic,
inasmuch as HIV requires both CD4 and an appropriate coreceptor for in-
fection. The di¨erential expression of CCR5 and CXCR4 on immune subsets
clearly impacts the type of cells that are susceptible to the pathogenic con-
sequences of viral infection.
The ®rst indication that CCR5 and CXCR4 are di¨erentially expressed on
subsets of CD4
lymphocytes comes from studies by Bleul et al. (1997) show-
ing that CCR5 expression is largely restricted to CD45RO
memory subsets
whereas CXCR4 expression is more skewed toward CD45RA
naive subsets.
These observations were subsequently con®rmed by others (Lee et al., 1999b;
Mo et al., 1998) and substantiated by evidence showing that, although R5-using
viruses mainly infect memory CD45RO
cells in vivo and in vitro, infection of
CD45RA
naive T cells is usually via syncytium-inducing, X4-using viruses
(Blaak et al., 2000; Ostrowski et al., 1999). The preferential infection of mem-
ory cells in vivo by both X4 and R5 viruses and of naive cells by X4-using
viruses closely parallels the di¨erential cell surface expression of CCR5 and
CXCR4 ( Blaak et al., 2000). Importantly, the frequency of X4 virus-infected
CD45RA
naive cells correlates with CD4
T-cell decline ( Blaak et al., 2000),
giving an example of how multiparametric FACS analysis of coreceptor ex-
pression in the context of immune system markers may shed light on the im-
munopathogenesis of AIDS.
Although coreceptor expression levels are an important variable governing
viral infectivity, expression levels alone do not necessarily predict the extent of
viral replication in vivo. For example, acute activation of peripheral blood
