The present study provides new insights into the functional role of IDO in the
clade-specific Tat proteins modulation of the kynurenine pathway and production of
IDO in primary astrocytes. However, there are no reports on the molecular mechanisms
of IDO expression by HIV-1 clade B and C Tat protein. We demonstrated that clade B
Tat increased IDO mRNA expression compared with clade C Tat. These findings are of
considerable interest, because not only IDO mRNA, but also levels of protein,
enzyme, and kynurenine are less compared to clade B Tat.
It has been shown that different sequence and genetic polymorphisms in HIV-1 may lead
to differential expressions of HIV-associated dementia (HAD). HIV-1 clade B is
predominant in North America, Western Europe, and Australia, whereas HIV-1 clade C
is common in Africa, Latin America, and Asia.
Current estimates place the prevalence of HAD in the United States and
Western Europe at 10–20%.
In India, several studies demonstrate that the prevalence of HAD is just over
HAD appears to be most common in HIV-1 clade B
infection prevalent areas but not HIV-1 clade C infection prevalent areas,
suggesting that the prevalence of HAD may be correlated with the difference in
subtypes of HIV-1.
HIV-1 infection activates glial cells to overstimulate IDO, leading to depletion of
tryptophan and increased production of kynurenine, an intermediate of the end
product quinolinic acid, to enhance brain cell death.
Increased IDO activity in the frontal cortex of HAD patients has already been
Moreover, previous studies have
shown that elevated levels of the neurotoxin quinolinic acid have been consistently
found in the cerebrospinal fluid (CSF) of HIV-infected patients with HAD,
suggesting that quinolinic acid plays a major role in the pathogenesis of
To our knowledge there are no previous reports on the molecular mechanisms of IDO
expression following exposure to HIV-1 clade B and C Tat proteins. The present study
provides new insights on HIV-1 clade differences by studying the modulation of the
kynurenine pathway and IDO production in primary human astrocytes. We demonstrated
that HIV-1 clade B Tat increased IDO mRNA expression as compared to HIV-1 clade C
Tat ( and ). Furthermore, our results also demonstrated that the induction of IDO
activity and the levels of kynurenine were significantly higher with HIV-1 clade B
Tat than with HIV-1 clade C Tat ( and
). Although clade C Tat showed a low level
of kynurenine, there was a lack of significant induction of IDO gene expression.
Our results suggest that HIV-1 genetic variations play critical roles in influencing
HIV-1 infection and differentially modify disease progression.
The sequential differences in structural and functional aspects of HIV-1
clade C Tat compared to HIV-1 clade B Tat occur especially in the dicysteine motif,
binding to CCR2 receptors,
and the capacity to induce proinflammatory cytokine TNF-α and MCP-1.
Recent studies have shown that HIV-1 clade B Tat protein induces more
secretion of TNF-α, IL-6, and the chemokine coreceptor CCR5 compared to
HIV-1 clade C Tat
. We consistently found that HIV-1 clade B Tat significantly increased MCP-1
compared to HIV-1 clade C Tat (),
therefore raising the possibility that these factors may be involved in the observed
effects. It is possible that these factors contribute to the differential modulation
of kynurenine and IDO activity. There are several stages at which IDO expression can
be regulated at transcriptional, translational, and posttranslational
which may have an impact on IDO enzyme activation.
It has been shown that various proinflammatory cytokines such as IFN-γ,
TNF-α, and IL-1β increase IDO activity.
These cytokines are also known to rapidly activate signaling pathways that
This suggests a possible role for neuropathogenic molecules regulated by IDO
activation in HAD. Further studies are needed to fully define the mechanistic effect
of HIV-1 clade B- and C Tat-induced IDO enzyme activation.
Our results show that HIV-1 clade B Tat-induced activation of IDO proteins () was associated with a concomitant
elevation of kynurenine (). In contrast,
differences in protein modification in intracellular signal transductions results in
differential IDO mRNA expression and enzyme activations in Tat clade B- and Tat
clade C-exposed cells. However, the mechanisms underlying these alterations and
their protein modifications are yet to be elucidated. Because the main observation
in this report is that Tat clade B and C both increase kynurenine concentrations but
yet result in different patterns of IDO gene expression, these two different clades
may have a distinct in vitro
mechanism. Our studies show that the
Tat protein expression level did not correlate with either the mRNA expression or
enzyme levels in Tat clade C (, , and ).
Previous studies indicate that without IDO enzyme activation, the IDO protein was
still expressed in mouse splenic cells and human DC.31,32
Thomas et al.
indicated that IDO activity and protein expression
are not dependent on IDO mRNA expressions but are limited by the cell heme protein
availability for IDO activity.
In our study of CD115 astrocytes treated by Tat B, we observed a high degree
of expression of the IDO gene and protein and a significant increase in IDO enzyme
activity and kynurenine concentration simultaneously, whereas astrocytes treated by
Tat C demonstrated that increasing only IDO enzyme activity and kynurenine
concentration resulted in no significant changes in terms of gene or protein
expression. Overall, the data provide evidence of a connection between IDO gene
expression and enzyme activation, enhancing the formation of kynurenine
concentrations in Tat clade B-exposed cells.
In conclusion, HIV-1 Tat protein treatment increased IDO expression and activation in
primary human astrocytes. Furthermore, HIV-1 clade B Tat could induce more
kynurenine as compared to HIV-1 clade C Tat in human primary astrocytes. Based on
these results, this might be the reason why HIV-1 clade B has been suggested to be
involved in neuropathogenesis in terms of the development of HAD. So far it has been
demonstrated that an increase in IDO enzymatic activity in the brain could lead to
the enhanced production of neurotoxins,
resulting in neurocognitive dysfunction and HAD.