A minor subset within CD11c+ dendritic cells is primarily responsible for peripheral Treg expansion

FOXP3+ Tregs play a major role in tolerance maintenance in the periphery. Most of FOXP3+ Tregs are thymus derived. Thymus is a specialized lymphoid tissue that generates novel T cells from their bone marrow precursors. The question whether naive T cells could convert into FOXP3+ Tregs in the periphery has not been settled. 

It is believed that a "steady-state" condition favors FOXP3+ Tregs conversion in the periphery. However, a concept of "steady-state" is an arbitrary one, defined as absence of deliberate immunization or experimentally observed infection. In fact, whether "steady-state" truly exist is an open question as well.

Why this matters? Almost everyone agrees that in absence of so called "steady-state" naive T cells would convert into effector T cells rather than into FOXP3+ Tregs following antigen recognition. For example, if one wants to generate new FOXP3+ Tregs specific for particular antigen to treat autoimmune diseases, this task would be almost impossible to achieve if condition of "steady-state" does not actually exist in the body [from T cells' "point of view"]. 

Also, what cell types are responsible for that supposed FOXP3+ Tregs conversion? A new study in Immunity clarified this question somewhat. It showed that even in "steady-state" condition only minor subset of DCs within CD11c+ population defined by DEC205/CD8 expression were responsible for FOXP3+ Tregs "conversion" in T cell-replete mice [which harbor endogenous FOXP3+ Tregs].

For this study, the authors have used chimeric anti-DEC205 Ab [or anti-CD11c chimeric Ab as a control] that incorporate antigen of interest [MOG or OVA]. When injected into mice chimeric anti-DEC205 Ab, but not control, could "convert" naive MOG or OVA-specific T cells into FOXP3+ Tregs.

It appeared that DEC205+ CD11c+ DCs were also primarily BTLA+ and its expression were required for FOXP3+ Tregs induction.

Interestingly, the authors proposed that BTLA to HVEM signaling in naive T cells up-regulated CD5 and permitted FOXP3+ Tregs conversion even in presence of inflammatory cytokines such as IL-4 and IL-6.  

However, in my view, such mechanism of FOXP3+ Tregs conversion even in presence of inflammation sounds counter-intuitive. Wouldn't it also induce FOXP3+ Tregs conversion from naive T cells specific for nonself antigen derived from pathogens during inflammation? Otherwise, how can system make sure that only self antigens are presented by DEC205+ DCs? The authors could only admit that this tolerance mechanism somehow only affects "self and tolerizing antigens". Also, what about endogenous FOXP3+ Tregs in these mice? Is it possible that endogenous thymus FOXP3+ Tregs are involved in assisting in FOXP3+ Tregs conversion, rather than DEC205+ DCs doing it alone from scratch? If so, implications are very different.  

David Usharauli

Tetra-punch against solid tumors

Checkpoint blockade therapies has become a gold standard for cancer immunotherapy. However, only in minority of cancer patients did these antibody therapies show significant benefits. Many think that a multi-pronged approach to cancer therapy could tip the balance in favor of anti-tumor therapy.

For sure, data from mouse studies support this line of thinking. For example, this week Nature Medicine published a mouse study showing dramatic benefits of immunotherapy when four different approaches were combined: 

1. Anti-cancer Antibody (A)

2. Long-lived IL-2 (I)

3. Checkpoint PD1 inhibitor (P)

4. Cancer Vaccine (V)

Referred as AIPV this experimental tetra-pronged immunotherapy could clear an established solid tumors (melanoma, breast cancer, adenocarcinoma) in 75%-80% of mice.

Success of AIPV therapy depended mostly on CD8 T cell and NK cells.

Of note, however, frequency of IFN-γ⁺ CD8 T cells did not correlate with anti-tumor effectiveness.

Interestingly, through AIPV could induced endogenous anti-cancer antibodies that transferred protection in naive hosts against intravenous tumor inoculum, B cell deficient mice were still protected against tumors when immunized with AIPV.

Finally, AIPV protected against autochthonous [endogenously developed] tumor in Braf^CA Pten^loxPTyr::Cre^ERT2 mice.

In summary, this mouse study shows that multi pronged immunotherapeutic approach could significantly improve survival rate during cancer therapy. The authors claimed that "AIPV therapy was associated with minimal systemic toxicity, as mice did not show weight loss or substantial elevation in the amounts of liver enzymes in the blood".

Of course, it is difficult to compare outcome in mouse study versus human study. In humans, even single approach with anti-PD1 antibody frequently leads to lung or liver toxicity. Now imagine injecting cancer patients with 4 different immunotherapeutics. So, we have a long way to go before immuntherapy will show the same acceptable-level effectiveness in humans as it does in lab mice.  

David Usharauli 

   

FOXP3+ Tregs and allergic TH2 cells recognize mutually exclusive allergic proteins

This week journal Cell published new data about FOXP3+ Tregs that could significantly advance our understanding of immune regulation.  

One reason why it is so difficult to study human FOXP3+ Tregs is that we don't have a good surface marker that selectively identifies human FOXP3+ Tregs from that of conventional T cell memory. As a consequence, we can't easily study antigen-specific human FOXP3+ Tregs.

In this new paper, the authors proposed that expression of CD137 versus CD154 identifies FOXP3+ Tregs versus Tconv in antigen-specific stimulation assay. Using CD137 as a marker, the authors showed that presence of FOXP3+ Tregs specific for variety of nonself antigens, including allergens.

The vast majority of human antigen-specific FOXP3+ Tregs displayed memory CD45RO+ phenotype.

Interestingly, TCR Vbeta analysis revealed mutual exclusivity of FOXP3+ Tregs and Tconv memory.

Next, when analyzing T cells from allergic individuals the authors first confirmed that indeed allergic individuals harbor significantly more allergen-specific TH2 cells.   

However, surprisingly, the authors found that allergic individuals harbor normal numbers of allergen-specific FOXP3+ Tregs.  

To understand this finding, the authors conducted more stringent allergen-specific assay by culturing T cells with individual allergen protein rather than the whole allergen extract as customary (the whole extract contains several related proteins). Indeed, this modified assay showed mutual exclusivity of antigens recognized by human FOXP3+ Tregs and allergic TH2 cells.   

In summary, this paper showed that unlike conventional wisdom, failure of FOXP3+ Tregs to control TH2 cells is antigen-specific. In essence, allergic individuals have specific "holes" in FOXP3+ Tregs repertoire that do not allow them to prevent allergen-specific naive T cells differentiation into allergic TH2 cells. This is first checkpoint. However, simple absence of allergen-specific FOXP3+ Tregs is not sufficient to induce allergy. Second checkpoint for allergy development requires additional factors, not yet fully understood, that promote TH2 response.

David Usharauli