Hypocretin-specific T cells mediate autoimmune sleeping disorder narcolepsy

Narcolepsy is a sleeping disorder characterized by "excessive daytime sleepiness, cataplexy, hypnagogic hallucinations and sleep paralysis". In recent years several studies provided evidence suggested that etiology of narcolepsy could be autoimmune in nature. Such conclusion has become more mainstream especially following observed relationship between 2009 pandemic flu vaccine, Pandemrix (from GSK), vaccination and development of sleeping disorder in a subset of vaccine recipients. In one study, it was found that Pandemrix but not Focetria flu vaccine (they differ in only one amino acid from each other) caused anti-flu antibody production that cross-reacted with hypocretin receptor.

Now, new study in Nature went further and showed that individuals suffering from narcolepsy, but not healthy controls, harbor hypocretin-specific T cells.

Antigen-specific T cells are rare so to reveal their presence the authors first non-specifically amplify T cells from PBMCs and after considerable expansion tested on hypocretin peptide pool pulsed autologous B cells. Majority of individuals with narcolepsy showed reactivity in this assay.

Single cell analysis showed that hypocretin-specific T cells consist of several clones (up to 30 different clones).       

Interestingly, most of T cells specific for hypocretin were HLA-DR restricted, rather than HLA-DQ as earlier association studies would have predicted.

Also, notably, hypocretin-specific T cells did not reacted with autologous B cells (or monocytes) pulsed with hypocretin protein suggested that this in vitro pulsing assay could not recapitulate in vivo hypocretin protein processing pathway. 

Of note, the authors showed that hypocretin-specific T cells did not cross-react with pandemic flu peptide pool or vaccine (influvac). However, they did not use Pandemrix vaccine here so these results are not conclusive. 

In summary, this study showed that narcolepsy could indeed be a bona fide autoimmune disease. As in many other autoimmune diseases exact molecular events that initiate them are yet to be discovered.

posted by David Usharauli 

Nucleoprotein in Flu vaccine mimics brain protein and may cause autoimmune sleepiness

Narcolepsy is a chronic neurological condition characterized by excessive daytime sleepiness. It causes are unknown. It is suggested that deficiency or inhibition of endogenous brain peptide called hypocretin (orexin) and produced by hypothalamus could recapitulate narcolepsy's symptoms.

New study in Science Translational Medicine suggested that immune cross-reactivity between Flu vaccine component and brain's hypocretin receptor could have caused autoantibody mediated narcolepsy following 2009 Flu H1N1.

First, the authors showed that nucleoprotein (NP) from flu vaccine Pandemrix (that caused narcolepsy) and NP from flu vaccine Focetria (that did not cause narcolepsy) have sequence similarity to human hypocretin receptor. These two NPs differed in only 1 amino acid from each other (isoleucin vs. methionine).

Next, the authors showed that serum samples from Pandermrix cohort, but not from Focetria cohort, reacted with human hypocretin receptor in a sensitive cell-based assay (note high reactivity in healthy Finish group). 

Importantly, the authors showed that this immune reactivity of Pandemprix vaccinated serum samples to human hypocretin receptor could be inhibited by short Flu NP peptides  (both from Pandemprix and Focetria) and or by hypocretin receptor (positive control). It suggested that at the antibody level, there is no difference between NP from Pandemrix or Focetria. The authors could not analyze T cell part.   

In summary, the authors suggested the following hypothesis: high level of NP protein in Pandemrix flu vaccine (compared to Focetria) could have induced robust anti-NP antibody titre that later "somehow" have managed to cross human brain blood barrier (BBB) and caused autoimmune narcolepsy is HLA susceptible individuals. However, it is not clear how autoantibody could have got access to hypothalamic cells expressing hypocretin receptors.

Vaccine are important for human health. However, human HLA diversity and natural mimicry at the protein and peptide level could produce autoimmunity in certain individuals.  

David Usharauli