Immunomodulation and Influenza Infection

The immune system provides a necessary response to infectious agents but is not always successful in eliminating the threat posed by such organisms.  Although some viruses are directly cytopathic and cause cell injury or death, others cause cytokine dysregulation resulting in overwhelming inflammatory processes, which can be more destructive than the virus itself and lead to death.  Such is the case with the Influenza A (H5N1) virus, a purely avian influenza virus, which was introduced into the human population of Hong Kong in 1997, causing a case-fatality rate of 33 percent, primarily due to pneumonia resulting from cytokine dysregulation [1], [2]. 

In the immune system, the endocannabinoid system is defined by the presence of endogenous ligands to the CB1 and CB2 cannabinoid receptors, members of the G protein-coupled receptor family, which are primarily responsible for attenuating, but not preempting, an immune response.  It has been established that both endogenous and exogenous ligands for the CB2 receptor are immunomodulators.  They lead  primarily to a downregulation of Th1 cytokines and chemokines and an upregulation of Th2 cytokines, which are antiinflammatory [3].  A study involving both occasional and regular use of smoked marijuana showed that IL-2 was downregulated, while IL-10, a Th2 cytokine and TGF-? , an immunosuppressive cytokine, were upregulated [4]. 

The anti-oxidant properties of the cannabinoids have been well described.  When compared to cigarette smoke, which increased the release of superoxide from macrophages during phagocytosis, marijuana smoke reduced its release by almost half, even when compared to the control cells during phagocytosis [5]. These anti-oxidant effects are mediated by a non-receptor-mediated pathway and are related to the phenolic structure of the classical cannabinoids, since the aminoalkylindole derivative, WIN-55,212-2, did not prevent oxidative cell death in serum-deprived activated cells [6]. One of the pathways suggested, by which these effects are obtained, is through the inhibition of the redox-sensitive activation of NF-?B, which is required for the expression of iNOS and the production of nitric oxide [7].  However, the endogenous ligand, 2-arachidonoyl-glycerol (2-AG), caused an increase in nitrite production in LPS stimulated macrophages  [8].

Research into anti-inflammatory potential of the cannaniboid class of drugs has been directed primarily at diseases mediated by injury, allergens and the immune dysfunction of autoimmune disorders [9].  Allergy and infectious disease processes share many common pathways in the lungs, resulting in the influx of inflammatory cells and intraepithelial-stored mucosubstances (IM) [10], which are greatly diminished by the treatment of ?⁹-THC and cannabinol in an ovalbumin-sensitized mouse model [11].  This effect was mediated by the attenuation of IL-2, IL-5 and IL-13 mRNA expression in a CB2 dependent manner.  Others have noted that the inhibition of macrophage migration is CB2 receptor dependent and that THC markedly reduced the expression of  CCR2, the receptor for MCP-1, in splenocytes stimulated with TNF-? [12].

Drug Development

It has already been demonstrated that the classical cannabinoid structure acts as a platform for the development of ligands to the CB2 receptor, as well as possessing anti-oxidant properties with its phenolic structure.  Thus, it would be logical to use such a compound in experimental studies with known influenza strains to establish proof of concept.  In fact, a recently published article established that a THC-treated wild-type mouse, instilled with PR8 (H1N1), showed modest-to-no inflammatory cell infiltrate of the submucosa and alveolar airspace [13].  Immugen’s  two lead compounds, L759656 and L759633, developed by MerckFrosst in 1996 and for which no patents had been applied, are now being claimed for their utility in continuation and CIP applications for the treatment of influenza infections, among other indications.  These are classical cannabinoids, i.e., tricyclic phenolic compounds, which have extremely high affinity to the CB2 receptor at nano molar concentrations [14], [15).  It remains to be determined which non-receptor mediated activities are associated with the compounds.  As with other cannabinoids, the drug should be highly soluble in beta-cyclodextrins and administered as a nasal aerosol spray or metered dose inhaler to the lungs; both routes would avoid the first-pass effects through the liver.  However, it was recently determined that both compounds are metabolically stable after being cultured in human liver microsomes for 120 minutes [16].  The highly lipophilic nature of the drugs would ensure a slow release from fat stores in the body, which could result in dosing every other day [17].

Although the use of systemic steroids has proven useful for the treatment of a variety of viruses, which cause acute airway disease from bronchiolitis to asthma, it is probably contraindicated for the treatment of avian influenza [18].  However, unlike steroids, CB2 receptor ligands do not affect innate immunity, but rather modulate the immune response through signaling pathways to help restore homeostasis once a response has been initiated.  Since it has been observed in occasional users of marijuana that IL-10 and TFG-? are upregulated, while IL-2 is downregulated (Pacifici et al.), it is possible that the drug could be used prophylactically and therapeutically to blunt a deleterious immune response in communities that are being threatened by H5N1.