Biology Reference
In-Depth Information
requirements for CD4 and coreceptor levels ( Kozak et al., 1997; Platt et al.,
1998). Platt et al. (1998) determined that, where CD4 levels were ``limiting''
(< 10
4
), levels of CCR5 below a threshold of 1±2 10
4
molecules signi®cantly
a¨ected the e½ciency of infection. However, when CD4 levels were high
(4.5 10
5
), minimal levels of CCR5 2 10
3
supported maximal viral infec-
tivity. In addition, primary X4 viruses appear more dependent on CD4 levels
than lab-adapted strains ( Kabat et al., 1994). Because CD4 antigen density on
PBMC is quite high (between 10
4
and 10
5
molecules per cell (Lee et al., 1999b)
whereas coreceptor levels are typically less than 10
4
per cell, it is likely that
endogenous coreceptor levels (Hladik et al., 1999; Lee et al., 1999b; Reynes et
al., 2000) will have a greater impact on the e½ciency of viral entry. In addition
to the slower disease progression noted in HIV-infected ccr5D32 heterozygotes,
presumably due to lower expression of CCR5 on lymphocytes and lower viral
loads (see above), there is evidence to indicate that CCR5 expression on cell
types other than immune cells may also impact on disease pathogenesis and
survival. For example, one study showed an under-representation of ccr5D32
genotype in patients with AIDS dementia complex (van Rij et al., 1999). Be-
cause CCR5 is required for viral replication in macrophages ( Rana et al., 1997)
and microglial cells in the central nervous system (Albright et al., 1999; He et
al., 1997; Shieh et al., 1998), this suggests that expression levels of CCR5 can
directly a¨ect the size of a macrophage-tropic reservoir of HIV-1 in the brain.
Indeed, we have quanti®ed various chemokine receptors on microglial cells
(CCR5, CCR3, and CXCR4) and found that, despite the ample presence of
chemokine receptors with coreceptor activity, CCR5 is still the major core-
ceptor used in these cells (Albright et al., 1999). As such, this serves as another
example whereby careful quanti®cation of coreceptor expression has led to in-
sights regarding speci®c aspects of the viral pathogenesis.
CONCLUSION
The identi®cation of the cellular receptors used by HIV to infect cells has pro-
vided considerable insight into viral tropism and pathogenesis. However, full
understanding of the complex interactions between the virus and target cells in
vivo requires more accurate measurements of receptor density. Attempts to do
so are complicated by the dynamic manner in which these receptors are regu-
lated, the intricate network of chemokines and cytokines that can directly or
indirectly a¨ect receptor expression, and the fact that both CCR5 and CXCR4
exist in antigenically distinct conformations. The use of QFACS makes it pos-
sible to quantify receptor expression in an easy and reproducible manner, ob-
viating many of the experimental di½culties encountered by earlier studies. By
coupling this approach with virological and clinical data, it should be possible
to more fully understand the complex interactions between the virus and its
receptors.
