**9. Conclusion and future perspectives**

In conclusion, there is a significant correlation between the circulating number of lymphocytes TH and Tc, B cells and NK cells, and the μOR allele present, and this difference can be further increased in the presence of exogenous opioids either during clinical treatment or substance dependence, as the 118G allele affects the process of cell division arresting cells at the S or M phase of the cell cycle, or by modulating the action of cell division–linked secretion of stimulating cytokines/chemokine known to induce clone-specific and cell type–specific proliferation, because of the propensity of opioid receptor to heterodimerize and to selectively bias the subsequent ligand engagement/s with the dimerized/ oligomerized receptors. This "subliminal immune suppression" in G allele–bearing individuals can have far reaching impact on onset of diseases such as cancer and obesity (both have an element inflammation) and vaccination for infectious diseases and even dreaded diseases as cancer. This immune suppression will certainly lower the individual's risk for autoimmune disorders such as rheumatoid arthritis, lupus, etc. The relationship between the μOR-mediated cell signaling and impact of stimulation of MOR as partner receptors, which influences binding of the second ligand in immunocytes and thereby the outcome on immune cells function in mounting and regulating the immune response/s, needs more detailed molecular exploration.

*Immune Response Activation and Immunomodulation*

defense (innate immunity) and also help initiate specific defense mechanisms (adaptive immunity) by recruiting other immune cells such as lymphocytes. Where a pathogen is involved, monocytes are commonly preceded by neutrophils, which release a range of toxic agents designed to kill extracellular pathogens. The macrophages then clear out both the dead pathogens and the dead neutrophils. The process of recruitment of neutrophils and macrophages involves the activation of resident macrophages. Activated resident macrophages respond to local stimuli by producing cytokines that make the endothelial cell surface more sticky (through induction of increased expression of cell adhesion molecules such as P-selectin) and chemokines, that promote and direct migration of inflammatory cells. Macrophages treated with LPS express high levels of cyclin D2 [91]. The CD14 and TLR signaling funnels into the activation of NF-κB, AP-1 and IRF3 pathways via the ERK1/2 and MEK1/2 channel, which is also activated by μOR signaling. Both signaling pathways funnel into the MAPK cascade and hence effect the activation of NFκΒ. Preliminary data from our study using both A549 and HepG2 cells show a significant decrease in LPS-induced cell division in the simultaneous presence of ligands for μOR. Cell cycle arrest was observed primarily in the S and the M phase of cell cycle with a significant decrease under conditions of costimulation with both the receptor agonists. Cell cycle arrest observed as a result of receptor coactivation needs to be further explored as it can have important therapeutic implications as on one hand a particular signaling pathway or end point is associated with a therapeutic response, such as analgesic effect of opioids, while on the other hand it is associated with unwanted side effects, namely cell cycle suppression on exposure to LPS and opioids together. Receptor heteromers, with their allosteric properties, give rise to a new kind of pharmacological target. The ability of one of the protomers to act as an allosteric modulator of the second protomer in the receptor heteromer gives the possibility of finding selective ligands for the protomer acting as conduit of the allosteric modulation [73]. As opioid drugs are used in more than one clinical condition understanding opioid receptor interactions with other GPCRs on the surface of different cell types can prove to be very beneficial in managing adverse, unwanted effects of these important therapeutic molecules. The immunomodulatory effects of morphine have been characterized both in animal and human studies and was found to decrease several functions of both natural and acquired immunity, interfering with important intracellular pathways involved in immune regulation [92]. Opiates namely morphine, heroin, fentanyl and methadone all induce immune-suppression and affect both innate and adaptive immunity defining a role of μOR in these functions.

**8. Association of OPRM1 functional alleles with immune cell function**

Sharad et al*.* [93] used a genetic approach to correlate a functional *OPRM1* gene polymorphism with known action of opiates on immunity and undertook a prospective study to understand the relationship of the 118G variation with the amount of exogenous opiates consumed and correlated the immunosuppressive effects of exogenous opiates with the *OPRM1* allele type. They studied the immune status of opiate users by measuring serum Ig (IgG and IgA) levels, in association with specific *OPRM1* genotype, and confirmed that the mean circulating levels of Ig were significantly lower in opiate users when compared with levels in cohort controls. Among opiate-dependent subjects, individuals with AA genotype were found to have the lowest levels of circulating immunoglobulins, both IgG and IgA (p = 0.0001), while the AG genotype carrying individuals had a higher level of both immunoglobulins. The homozygous GG genotype was in between the AA and AG genotypes. Alternatively, in opiate naïve subjects, the AA individuals showed the highest titers of

**92**

*Immune Response Activation and Immunomodulation*
