Adaptive Th2 cells can double as innate cells during allergy and helminth infection

Antigen specificity is a dominant feature of adaptive immune system. However, on occasions, fully differentiated effector T cells do show TCR-independent responsiveness to cytokines. Whether this is a good or bad thing is not easy to tell. 

For example, lets examine new paper published in Nature Immunology. This study, led by Bill Paul at National Institutes of Health (NIH), showed that in absence of TCR signalling, fully differentiated Th2 cells could respond to type II priming cytokine IL-33 and secrete type II effector cytokine IL-13. These results could impact our understanding of allergy therapy.    

For this study the authors had used a dual-reporter mouse model, called 4C13R, where both cytokine IL-4 and IL-13 are genetically labeled. Initially, the authors showed that adoptively transferred in vitro differentiated Th2 OT-II transgenic T cells, on 4C13R background, could secrete IL-13 (but not IL-4) in response to cytokine IL-33 (in combination with STAT5 activator IL-7 or TSLP).

Similarly, the authors observed TCR-independent IL-13 secretion by in vivo differentiated Th2 OT-II 4C13R cells to a Papain challenge (Papain is a protease and has been shown to induce type II immune response).

This TCR-independent secretion of IL-13 by differentiated Th2 cells was driven by IL-33.

Since type 2 innate lymphoid cells (ILC2) are known to respond to IL-33, the authors analyzed secretion of IL-13 by both Th2 and ILC cells after Papain challenge (here, mice were initially exposed to N. brasiliensis, a type II activator helminth). Both population could respond to Papain by secreting IL-13 (but not IL-4).

Similar results were obtained with another allergen, house dust mite extract (HDM).

Further experiments with RAG2-IL-2Rγ DKO hosts, which lack endogenous T and ILC2 cells, revealed that adoptive transfer of in vitro differentiated Th2 cells was sufficient to induce eosinophilia in lung tissue upon HDM exposure (allergen exposure).

The authors reasoned that in physiological situations initial antigenic priming would increase the number of Th2 cells to the level comparable to that of ILC2 and this would play a significant role in host protection or response to type II irritants (worm or allergen). Indeed, the authors showed that Th2 cells developed after first helminth infection could provide partial host protection to a second helminth infection, even in absence of ILC2 cells (in RORα bone marrow chimera).

In summary, these results suggest that adaptive immune cells can take the function of innate cells when necessary. Here, IL-13 appears to be a dominant cytokine that drives type II immune effector function (in 1990s ad early 2000s, the prevalent hypothesis positioned IL-4 in the center of type II immune effector response).

Such TCR-independent responsiveness has important implications for treatments of type II immune disorders such as allergy or dermatitis. For example, antigen (TCR ligand) desensitization protocol for allergy treatment may not provide full protection against allergic reaction due to IL-13 secretion (since it can be secreted independent of TCR signaling). On the other hand, blockade of IL-13 and IL-33 signaling may represent clinically relevant therapy (alone or in combination with antigen desensitization).  

David Usharauli
   

Bypassing "do not eat me" signal could unleash anti-tumor T cell response

CD47 receptor on healthy cells serves as a "do not eat me" signal when engaged by its ligand, SIRPα, on phagocytic macrophages. As cells ageing they express less of CD47 and become targets for scavenger macrophages. In 2013, a study was published that showed that anti-CD47 antibody therapy could prime anti-tumor CD8 T cell response via cross-priming phagocytic macrophages. 

This time, however, a new paper in Nature Medicine suggested that anti-tumor effect of anti-CD47 antibody therapy is mediated via dendritic cell-enabled cross-priming of CD8 T cells involving intracellular DNA recognition by STING-IFNα pathway. Lets examine the data.

First, the authors showed that the growth of both lymphoma (A20) or solid tumors (MC38) were inhibited in wild-type mice after injection of mouse anti-CD47 monoclonal antibody.

This anti-tumor effect of anti-CD47 antibody was dependent on CD8 T cells.

In addition, using CD11c-DTR BM chimera, the authors showed that anti-tumor effect of anti-CD47 antibody was dependent on CD11⁺ dendritic cells.

Further experiments revealed that IFNα singaling in CD11⁺ dendritic cells played an important role.

Finally, the authors showed that tumor growth in mice deficient for intracellular DNA sensing signaling pathway, called STING (tmem173), was not inhibited by anti-CD47 antibody.

In summary, these results suggest the following scenario: anti-CD47 antibody injection facilitates uptake of tumor DNA and associated antigens by DCs. Within DCs, tumor DNA activates STING-IFNα pathway and allows anti-tumor CD8 T cell priming.

From this study it is not clear whether anti-CD47 antibody has any direct cytotoxic effect on CD47⁺ tumor cells. Also it is not clear how tumor antigen and DNA is delivered to DCs. The authors did not mention anything about any side effects of anti-CD47 therapy either (CD47 is expressed ubiquitously).

There is one figure in this paper that is problematic. While in all other figures anti-CD47 antibody treatment alone has a clear anti-tumor effect in wild-type mice, in Fig. 6a, injection of anti-CD47 antibody alone in wild-type mice shows no anti-tumor effect at all (A20 cells and antibody injection on day 12, 17).

I could not find any explanation in this paper for this major discrepancy. This could question the reliability of anti-CD47 treatment.

David Usharauli  

Treg-specific expression of amphiregulin prevents immunopathology during viral infection

Foxp3⁺ T cell deficiency leads to lethal immunopathology in mice and severe organ pathology in humans (IPEX syndrome). Over the past 20 years numerous mechanisms of action of Foxp3⁺ T cells have been described. In fact, this number is so numerous that it wouldn't be a "heresy" from my part to claim that no single Foxp3⁺ T cell phenotype could explain it. This leaves us with the hypothesis that multiple versions of Foxp3⁺ T cells exist, each of them using selective path for immune regulation and suppression.    

With this view in mind, it was useful to read a new paper from Rudensky's lab published in journal Cell this week that provided evidence showing the role of Foxp3-positive T cell-specific amphiregulin in preventing excessive tissue pathology during high dose of viral infection.

First, the authors verified that amphiregulin, an epidermal growth factor family member, was expressed by Foxp3⁺ T cells.

Next, the authors showed that amphiregulin deficient Foxp3⁺ T cells displayed normal suppressive functionality when transferred into T cell-deficient host.

Further experiments revealed that amphiregulin deficiency in Foxp3⁺ T cells did not modify the host immune response to intranasal flu infection (mouse PR8 model).

However, the authors noticed that there was excessive lung tissue damage in response to high (but not low) dose of flu infection in the hosts with Foxp3⁺ T cell-specific deletion of amphiregulin.

Finally, the authors showed that amphiregulin up-regulation was primarily restricted to IL-18R⁺ Foxp3⁺ T cells.

In summary, these data tend to reinforce the idea that Foxp3⁺ T cells could function in a tissue selective manner and are most likely controlled by tissue environment as suggested by Polly Matzinger and Tirumalai Kamala. This could explain why there are so mechanisms of suppression by Tregs, at least one mechanism for each tissue.

What are some of the weaknesses of this study? First, injection of amphiregulin to hosts with Foxp3⁺ T cell-specific deletion of amphiregulin would have been useful. Second, data regarding IL-18R deficient Foxp3⁺ T cells could have provided in vivo confirmation for this study (the authors simply mentioned that they have done experiments with bone marrow chimera to test the role of IL-18R in amphiregulin up-regulation and that it supported their conclusions).

Why is this study important? Because without our mastery of Foxp3⁺ T cell biology we will not be able to make any major, predictable advances in treating human immune related conditions (cancer, allergy, autoimmune conditions).

David Usharauli