Adjuvant activity of cholera toxin relies on endogenous microbiota-dependent Nod2 activation

Cholera toxin (CT), derived from Vibrio cholerae, is still widely used as a potent mucosal adjuvant in experimental [rodent] immunology. CT binds GM1 ganglioside receptor and activates cyclic AMP (cAMP) production. This mechanism is thought to stimulate T_H2-associated cytokines and robust IgG₁ production. However, toxicity of CT prevents its implementation in the clinic (it is not uncommon that scientists are studying active molecules that have no clinical use, but all "in hope" that "the mechanisms underlying its [CT's] potent adjuvant activity may lead to the development of nontoxic and effective adjuvants for mucosal vaccination."

This time, new study in Nature Medicine showed that CT's adjuvant activity relied on Nod2 signaling initiated by endogenous microbial flora.

Initially, the authors reported that nasal or oral immunization of germ-free (GF) mice [that harbor no endogenous microbiota] with protein antigen + CT yielded reduced levels of antigen-specific IgG₁ and T cell effector differentiation. Similar results were obtained with antibiotic-treated mice.

Effect of endogenous microbiota on CT's adjuvant activity could be replicated in Ripk2 (the adaptor required for Nod1 and Nod2 signaling) or Nod2 deficient mice, but not in MyD88-KO or Nod1-KO mice.

It is known that Nod2 recognizes peptidoglycan molecules that contain muramyl dipeptide (MDP). Indeed, GF mice immunized with a combination of protein Ag + CT + MDP produced high level of antigen-specific IgG₁, suggesting the role of MDP in promoting CT's adjuvanticity.

Finally, the authors found that several members of MDP-rich endogenous microbiota (i.e. Staphylococcus sciuri) promoted CT's adjuvant activity in GF mice in a Nod2-dependent manner.

In summary, this study indicates that adjuvanticity of cholera toxin relies on Nod2 signaling triggered by MDP-rich endogenous microbiota.

David Usharauli

Type II immunity is fueled by ILC2-specific arginase-1

In recent years much attention has been focused on group of innate cells called innate lymphoid cells (ILCs). There are group 1 (T_H1 behavior), group 2 (T_H2 behavior) and group 3 (T_H17 behavior) ILCs, so far.

This week, new paper in Nature Immunology from David Artis lab (extremely prolific research lab) showed that ILC2-specific arginase-1 (Arg1) contributes to lung allergic inflammation.

Arg1 is an enzyme that metabolizes the amino acid L-arginine. Most L-arginine metabolism occurs primarily in the liver, though immune cells can express it too during immune response (i.e. Arg1 activity is a key signature of alternatively activated macrophages). Here, fate-mapping study of Arg1-YFP-expressing cells in lungs of naive mice showed that Arg1 was mostly expressed by ILC2.

Papain challenge (T_H2 trigger) induced expansion of Arg1⁺ lung ILC2.

Using genetic mouse model where Arg1 is selectively absent in ILC2, Arg1^ΔILC, the authors showed that Arg1 deficiency impaired ILC2 expansion and allergic lung inflammation upon papain challenge.

Finally, the authors showed that lung allergic inflammation was not impaired if Arg1 was missing from macrophage lineage (Arg1^ΔLyz2), implying dominant role of ILC2-derived Arg1 in papain induced lung allergic inflammation.

In summary, this study revealed ILC2-specific role of Arg1 in promoting type II immunity in response to T_H2 triggers (papain and helminth parasite N. brasiliensis).

David Usharauli

New genetic model separates DC's innate and adaptive functions

The other day I reviewed an article where the authors used novel genetic model that could selectively delete classical/conventional DCs (cDC). It appears that this novel model is gaining popularity among scientists who are using it to re-define the role of classical DCs within immune system. I would remind you that prior to this model, people have used CD11c based genetic modification to assess DCs function. However, CD11c is expressed by other cells as well that could have influenced prior results.

In this new study published in Journal of Experimental Medicine, the authors led by Michel Nussenzweig, showed that cDC-selective deletion of MHCII leads to microbiota-driven gut inflammation.

This new genetic model is based on modification of gene called Zbtb46 (zDC) that is expressed in classical/conventional DCs (cDC), but not in monocytes, macrophages, and other myeloid cells (fate-mapping study). 

To separate cDC's innate and adaptive functions, the authors crossed zDC^Cre with MHCII^fl/fl mice to generate zDC^ΔMHCII mice that lack MHCII selectively on cDCs. cDCs from zDC^ΔMHCII mice showed normal "innate" function,

and as expected cDCs from zDC^ΔMHCII mice did not activate CD4 T cells upon antigen challenge.

Interestingly, zDC^ΔMHCII mice did not show any gross abnormality, except gut inflammation that was gut flora depended.

Finally, the authors showed that germ-free zDC^ΔMHCII mice colonized with defined flora produced reduced levels of T_FH cells and IgA.

In summary, this study showed that "adaptive" function of cDCs is important to maintain healthy gut homeostasis by generating CD4 T_FH dependent gut flora specific IgA.

David Usharauli