Live, not dead bacteria, augments human antibody response via TLR8 - T follicular helper cell axis

Immune system protects against pathogens. In natural settings, live pathogens cause diseases. So, it is very intuitive to think that immune system could have developed a specialized way to detect pathogen's "live signature". Few years back, Julie Magarian Blander's lab (former postdoctoral scientist in Ruslan Medzhitov' lab) published study showing that mRNA from live bacteria served as "live signature" they called vita-PAMP. 

Now, new paper in nature immunology from Leif Sander's lab (a former postdoctoral scientist in her lab and the first author of initial study) showed that in humans TLR8 may serve as a detector of vita-PAMP mRNA from live bacteria and augment antibody response. It is very nice study done primarily on ex vivo/in vitro human cells cultures (interestingly, Blander's lab also published this month new study about vita-PAMPs in mice in journal Immunity. But it is very messy and bloated study. Basically, in this case pupil outdid his [former] master).

Here the authors showed that live, replication-defective E. coli strain but not heat-inactivated dead one, could induce differentiation of human follicular helper T cells (T_FH cells).

These T_FH cells were functionally active inducing antibody-secreting plasmablast generation from B cells.

Live bacteria, not dead one, specifically induced IL-12p70 generation from human monocytes (unlike mouse, differentiation of human T_FH cells requires IL-12p70).

Indeed, antibody blockade confirmed a major role of IL-12p70 in generation of IL-21-producing human T_FH cells.

Stimulation of human monocytes with various TLR agonists showed that engagement of TLR8 (single-strand RNA sensor) with its agonist ligand (CL075 or R848) were responsible for vita-PAMP effect on IL-12p70 production.

Similar vita-PAMP effect of TLR8 signaling were seen for human IL-21-producing T_FH cell generation.

In summary, this study indicates that unlike LPS-derivative MPLA or CpG fortified vaccines, inclusion of TLR8 agonists, such as CL075, could augment antibody responses. While study is well done we need to keep in mind that it is produced by members of the 'same initial' group (Blander and Sander) who first reported vita-PAMP effect. We don't have analogous studies from other labs who can independently confirm these observations. 

posted by David Usharauli

  

Making sense of Foxp3 as a prognostic value in tumors

A transcription factor Foxp3 controls functionality of T cell subset called regulatory T cells (T_regs). Seminal studies in mice clearly showed that Foxp3⁺ T cells play a dominant role in suppressing excessive immune response.

But human studies introduced an additional confusion in T_regs field. Unlike, mouse Foxp3, human Foxp3 is up-regulated in human naive CD4 T cells upon activation. In fact, human Foxp3⁺ T cell population is divided into Foxp3^low and Foxp3^high population. Foxp3^low population lack regulatory properties, whereas Foxp3^high population represents a bona fide T_regs cells.

However, such separation of Foxp3 marker within individual tumor clinical samples are not always possible and ordinarily cancer biopsies are grouped into Foxp3^hi or Foxp3^lo for Foxp3 mRNA total expression levels that are above or below the [arbitrary] median. So, it is not surprising that [total] expression level Foxp3 biomarker in tumor tissue does not correlate with overall survival.

In this regard, it is interesting to read a new paper in Nature Medicine showing that inclusion of two additional biomarkers, IL-12 and TGFβ1, could provide that differentiating power to transform Foxp3 expression into a prognostic factor in cancer diagnostics.

For this study the authors analyzed colorectal cancer samples. They found that cancer samples could be grouped into A and B types based on presence of two separate Foxp3⁺ T cell population: Foxp3^low and Foxp3^high population. Type A samples mostly have Foxp3^high population whereas type B samples have both Foxp3^low and Foxp3^high populations.

As expected Foxp3^low T cell population lacked suppressive [cell proliferation inhibition] quality and most likely represent conventional activated T cells.

Gene expression analysis of type A and B cancer tissue showed that type B were enriched with several cytokines such as IL-12, TNF-α and also TGFβ1. In vitro Foxp3 conversion assay showed that when combined with TGFβ1, interleukin-12 [but not TNF-α] could specifically increase naive T cell differentiation into Foxp3^low population (most likely by preventing their differentiation into Foxp3^high population).

Interestingly, when cancer samples were classified based on mRNA expression levels of Foxp3, IL-12 and TGFβ1, there were clear differences in patients survival between groups:

(1) as expected Foxp3 expression by itself had no prognostic value

(2) in samples with IL-12^loTGFβ1^lo expression, mRNA Foxp3^hi was associated with poor prognosis

and (3) unexpectedly, in samples with IL-12^hiTGFβ1^hi expression, mRNA Foxp3^hi was associated with a better prognosis.

Finally, the authors find that cancer samples with IL-12^hiTGFβ1^hi expression harbored more of gut microbe Fusobacterium nucleatum.

In summary, this study indicates following scenario:

(a) in colorectal cancers, detection of mRNA Foxp3 by itself has no prognostic value.
(b) however, cancer biopsies with Foxp3^hiIL-12^loTGFβ1^lo mRNA expression could mean poor prognosis, because such samples contain mostly bona fide T_regs cells.
(c) but, cancer biopsies with Foxp3^lo/hiIL-12^hiTGFβ1^hi mRNA expression could mean good prognosis, because such samples contain mostly Foxp3^low population or it has sufficient inflammatory milieu to override any suppression by bona fide T_regs cells.

David Usharauli