In this study, we report the novel finding that the stress hormone NPY is expressed at physiologic levels by adipose tissue macrophages and participates in obesity-induced adipose tissue inflammation. While macrophage NPY expression is induced by inflammatory signals and obesity, our results support an anti-inflammatory effect of NPY signals on macrophage and DC maturation. In vitro, NPY receptor blockade on macrophages/dendritic cells promotes M1 cytokine gene expression and DC maturation.Similarly, BMT in vivo experiments showed that the loss of Npy expression exclusively in hematopoietic cells increased CD11c+ ATMs, however, a significant decrease in cytokine production was not observed. The discrepancy in these observations likely relates to the timing of NPY exposure as well as the fact that in vivo experiments involve multiple cell lines that can respond and generate NPY (e.g. adipocytes and nerves). In vivo, NPY supplementation decreased M1 ATMs in lean mice by suppressing the levels of circulating Ly6Chi monocytes, an unexpected and novel mechanism linking stress hormones and inflammation.
Our observation that myeloid cells within adipose tissue are a regulated source of NPY production is consistent with other reports of NPY expression in non-neuronal cells. Macrophage NPY expression is evident in publicly available gene expression array data 
. NPY induction has been reported in stimulated immune cells 
, platelets and endothelial cells 
and in the lung 
. Given our observations, it is possible that macrophages in other important metabolic tissues (e.g. liver and brain) may secrete NPY and influence metabolic tissues. Preliminary studies did not identify significant Npy
gene expression in microglia however, stress or inflammatory stimuli may be required for significant Npy
induction, as observed in astrocytes 
. Our studies also suggest that macrophage NPY secretion can be regulated post-transcriptionally, and therefore mRNA expression techniques may be imprecise in certain contexts.
Our data supporting an anti-inflammatory function for NPY in macrophages, support the evidence that NPY has protective functions during chronic stress in the context of anxiolysis 
and appetite 
. Our studies show that this effect is primarily mediated by Y1, Y2, and Y5 receptors in macrophages as gene expression analysis failed to detect the expression of the Y4 receptor in macrophages (data not shown). NPY has also been shown to play a protective role in shock, sepsis, and autoimmune conditions 
. In models of endotoxemia, NPY can block monocyte induction and improve inflammatory measures 
Macrophage derived NPY may participate in the connections between stress and obesity. Clinical studies have shown that increased stress plays a role in weight gain and modifies the risk for diabetes, but the specific mechanisms that link stress pathways with insulin resistance remain unclear 
. Our results show an early induction of NPY in obese adipose tissue which may dampen the early inflammatory response to obesity. With prolonged exposure to HFD, the anti-inflammatory actions of NPY may be overwhelmed by the ongoing accumulation of M1 ATMs. While our results show an anti-inflammatory function of NPY in lean states and mild obesity, previous work suggests a pro-inflammatory role for NPY in response to the combined effects of stress and HFD 
. Differences in the levels of NPY induction and alterations in NPY receptor number and type may explain this dichotomy. This hypothesis is consistent with recent studies showing differential NPY receptor contributions to leukocyte activation 
. In addition, our BM chimera experiments demonstrate that ATMs are a source of NPY in fat, but may not be the sole source. Understanding how factors such as NPY may influence local inflammation is key given that macrophage content and activity within adipose tissue is an important factor for insulin resistance and risk for type 2 diabetes.
Our data support a model where NPY generated from ATMs has autocrine, paracrine, and systemic effects. NPY production by macrophages is increased by HFD and dampens M1 ATM activity. The NPY produced from ATMs may also activate NPY1Rs on pre-adipocytes and adipocytes contributing to a decrease in lipolysis 
, which may further decrease pro-inflammatory signals. At a systemic level, NPY generated from hypertrophied adipose tissue may try to suppress the production/trafficking of circulating classical Ly6chi
While we have observed an induction of CD11c+ ATMs with short term HFD exposure () we were not able to see a significant alteration in glucose metabolism with loss of Npy
in hematopoietic cells. This may be because inflammation is only one component of the development of insulin resistance and dominates with long-term high fat diet exposure 
. Since the metabolic phenotype of whole body NPY deficient mice is relatively mild and relates more to diet intake 
, we did not anticipate a substantial metabolic phenotype in our BMT experiment. In addition, the BM chimeras were generated on the 129SV background which is less prone to weight gain with high fat diet and relatively protected from insulin resistance 
The end result of NPY signaling is likely complex and highly dependent on the inflammatory microenvironment. Adding to the complexity, NPY can be cleaved by enzymes such as DPPIV (CD26), which is also expressed in macrophages 
, and change the pattern of NPY receptor activation. Our observations may explain why DPPIV inhibitors are capable of improving obesity-induced inflammation 
In conclusion, our findings suggest that macrophage derived NPY and its anti-inflammatory effects may serve to preserve normal adipose tissue function in the setting of obesity. Further evaluation of the triggers for this response in adipose tissue may lay the groundwork for novel treatment strategies. Future work is required to fully understand the range of signals that NPY can produce, but this study provides a unique mechanism by which ATMs may influence the adipose tissue environment.