(II) Evidence for altered expression of cell
surface molecules on Teff that are important for engaging with Treg
cells
Treg cells suppress via a variety of contact dependent and independent
mechanisms (Figure 1). Some of the key molecules shown to be involved in
promoting Treg suppression include CTLA-4, PD-1, CD39, CD73, PD-1,
PD-L1, LAG3 etc. Several lines of data show some of these molecules,
e.g. PD-1, PD-L1, CD39, CD73, LAG3 are altered in TB. PD-1 and PD-L1
expression is elevated in CD4+ T cells from TB
subjects compared to healthy controls [97]; CD39 and CD73 are
increased in lung parenchymal CD4+ T cells of Mtb
infected mice [98] and LAG3 is increased in granuloma of macaques
with active TB compared to those with latent TB [99]. However, the
function of these molecules has not been specifically studied in the
context of Treg suppression in TB. We provided some direct evidence for
a role for PD-1/PD-L1 and β chemokine/CCR5 interactions. A comparative
RNA seq analysis of Treg suppression sensitive
(HLA-DR- Teff) and suppression resistant (comprising
HLA-DR+ Teff + and
HLA-DR- Teff) cells isolated from subjects with
pulmonary TB showed elevated expression of PD-L1 and β-chemokines in the
suppression sensitive HLA-DR- Teff fraction (Figure
2). Also, blockade of PD-1/PD-L1 and β-chemokine/CCR5 interactions
resulted in loss of Treg suppression in these cells pointing to the
importance of these pathways in maintaining Treg mediated homeostasis in
TB (Figure 2). PD-1/PD-L1 interactions and β-chemokine/CCR5 interactions
have been previously implicated to play a role in Treg mediated
suppression [100, 101]. CCL3 and CCL4 secreted by Tregs serve as
chemoattractants for Teff cells. CD4+ Teff cells from
mice deficient in CCL3 and CCL4 fail to migrate and come in contact with
Treg cells [101]. Moreover, Tregs from type-1 diabetes patients are
deficient in CCL3 and CCL4 and this compromises their ability to
suppress [101].
Apart from differences in PD-L1 and β-chemokine levels (Figure 2), our
transcriptome analysis of Treg sensitive HLA-DR- and
suppression resistant HLA-DR+ cells identified several
additional cell surface markers (CD46, TRAIL, TRAF1, TRAF3, FASLG, CD30,
SEMA7A) (Figure 3), some of which have previously been implicated in
Treg function [102-105]. Engagement of complement receptor CD46
results in suppression of bystander CD4+ T cells via
an IL-10 dependent mechanism [102, 103]. CD46 cross-linking also
suppresses mycobacteria specific CD4+ T cell responses
[106]. TRAIL is a regulator of T cell activation; its absence leads
to autoimmunity and reduction in Treg frequencies while its presence
dampens Th1 responses and boosts Tregs [104]. TRAF1 inhibits Th2
differentiation [107]; TRAF3 controls proximal T cell activation
events and its absence in mice leads to elevated thymus derived Treg
cell frequencies [105]; FASLG is a marker for T cell activation and
is expressed on Th1 cells [108, 109]; Sema7a and CD30 have been
implicated in Th1 and Th17 differentiation [110, 111]. However, a
role for these pathways impacting T effector function in TB remains to
be elucidated.