What could be learned from vaccines that did not work?

It is >30 years since HIV discovery and we still don't have vaccine that can protect against it. Billions of dollars spent. And still no one could explain why HIV vaccines developed thus far are failing one after another (RV144 HIV-vaccine trial is one of those).

Here is what I think. Modern vaccinology is based on concept introduced 50 years ago. It is based on assumption that adaptive immune system (canonical B and T cells) is the main driver of protection. 20 years ago, after discovery of TLRs, there was a conceptual shift towards dendritic cell (DCs)-oriented vaccines. Nonetheless, the main focus of vaccine development still remains adaptive immune system (B cells, T cells and DCs). I think it is time to rethink our approach to vaccination. Adaptive immunity is one part of equation. There are growing lists of innate/innate-like cells that could play crucial role in next-generation of vaccines.

For example, following the RV144 HIV-vaccine trial, there was lots of head scratching. Some thought we need to use stronger adjuvants [compared to Alum]. However new study conducted on monkeys showed that "the higher vaccine immunogenicity of MF59 [adjuvant] does not directly translate to higher vaccine efficacy against SIV [monkey HIV] acquisition."

Moreover, this study found that higher percentage of SIV Env-reactive innate NKp44+IL-17+ cells  following alum-vaccine "were associated with a reduced risk of acquisition [of SIV in monkeys] in the multivariate analysis" (in other immune parameters MF59 was better than alum).

In summary, we need to acknowledge that the more learn about immune system the more we understand how little we know about it. 

David Usharauli

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

New molecule, BHLHe40, links adjuvant activity of pertussis toxin to T cell pathogenicity in brain inflammation

Multiple sclerosis (MS) is a human neuro-inflammatory disease of autoimmune nature. Mouse model of MS is called experimental autoimmune encephalomyelitis (EAE) and it's induction in mice depends on dirty little secret: to induce EAE, mice are injected not just with peptide derived from myelin oligodendrocyte glycoprotein (MOG35-55) emulsified in CFA (to activate T cells) but mice are also injected with the co-adjuvant pertussis toxin (PTX), an ADP-ribosylating exotoxin derived from Bordetella pertussis (PTX is a whooping cough toxin that has been shown to be necessary for MS induction in this mouse model). No one really knows how or what way PTX primes mice for MS/EAE induction or why we even needed it in the first place (by the way, next time you hear that another drug failed in clinical trial for multiple sclerosis you know now it is because mouse model is completely artificial).

In this regard, new paper in Journal of Experinental Medicine is of interest. The authors report that transcription factor basic helix–loop–helix family member e40 (BHLHe40) was required for adjuvant activity of PTX for EAE development.

Using BHLHe40^GFP mice the authors first showed that CD4 T cells expressing GFP (surrogate marker for BHLHe40) were enriched for effector T cells expressing cytokines implicated in EAE pathology (IFN-γ, IL-17, GM-CSF).

Next, the authors showed that ADP-ribosylating activity of PTX was necessary to augment GFP/BHLHe40 expression in T cells and to induce EAE.

Adoptive transfer of BHLHe40-deficient 2D2 T cells (that are specific for MOG peptide) confirmed that BHLHe40 expression was required for T cell pathogenicity in EAE.

In summary, this study showed that during EAE induction PTX sensitizes both innate cells (for IL-1 production) and adaptive immune T cells (for GM-CSF and IFN-γ production) via action of BHLHe40.

David Usharauli