The reasons for these divergent results are still unknown but as we understand more about the immune system in adipose tissue one could speculate several explanations for these discrepancies. One possibility is that microbiome differences between laboratories and between wild-type and knockout mice contribute to the difference in weight gain, as the microbiome has been to shown to significantly impact metabolism ABT-199 supplier and the development of obesity,[65] as well as iNKT cell development.[66] We have exchanged cages between non-littermate wild-type and iNKT knockout mice to reduce the impact of the microbiome. However, the reference standard is to use wild-type and knockout littermates to eliminate

the impact of the microbiome, which were used in some,[63, 67] but not most, of the studies summarized above. Another plausible explanation is the age of mice

in each study. In young mice, there is a substantial population of iNKT cells and fewer regulatory T cells in adipose tissue, and at 8–16 weeks, iNKT cells accumulate further but decline in old age, whereas adipose regulatory T cells greatly accumulate in old mice.[51] Therefore, it is plausible that iNKT cells may be more influential in younger mice, whereas in older mice it is the regulatory T cells that dominate and the role of iNKT cells, or lack of them may be less dominant. It is also possible that for some reason both wild-type and iNKT-deficient animal Oxymatrine colonies in different laboratories have a more Th1 or more Th2 bias among iNKT cells or other lymphocytes, or in some colonies, there is a compensatory MG-132 clinical trial mechanism when iNKT cells are absent from birth. Despite the divergent results using iNKT-deficient mice, other methods

to measure the effects of iNKT cells on obesity and metabolism are more consistent. First, over 14 independent studies have shown that iNKT cells (when measured accurately) are depleted in obesity, and all human studies have also found iNKT deficiency associated with obesity. Other immune cells that are shown to be protective in obesity,[52] such as regulatory T cells,[51] alternatively activated macrophages and eosinophils,[54] are depleted in obesity, whereas those that are shown to be pathogenic in obesity like CD8+ T cells[50] and classically activated macrophages[56] are increased in obese adipose tissue. Based on this comparison, which is not direct evidence and merely an association, iNKT cells appear to be part of the protective anti-inflammatory immune cell group that are lost in obesity as an inflammatory response takes over. More direct evidence comes from gain of function experiments, when iNKT cells are adoptively transferred into wild-type or iNKT-deficient obese mice or activated in wild-type obese mice. The majority of studies have shown that this has a positive impact on metabolic control and on protection against weight gain.