In this study, hypoglycaemia caused an acute and persistent pro-inflammatory effect, defined by changes in number, phenotype and function of monocytes combined with increased levels of various pro-inflammatory mediators. Although adrenaline responses were diminished after antecedent hypoglycaemia, there was no robust modifying effect of antecedent hypoglycaemia on these pro-inflammatory responses. Only the persistent response of circulating inflammatory markers seemed slightly decreased.
Our data are in line with earlier observations showing that hypoglycaemia acutely causes a range of pro-inflammatory responses (13–15), including increases in the number of immune cells (4, 5, 16), a phenotypical shift towards more pro-inflammatory non-classical monocytes and increases in cytokine production and release of pro-inflammatory proteins. We previously showed that many of these effects occur in both people with or without type 1 or type 2 diabetes, irrespective of level of glucose control or reported hypoglycaemic awareness (4, 5, 16). The present study now extends these findings by showing that these pro-inflammatory effects are largely unaffected by recent, i.e. previous day, antecedent hypoglycaemia.
In line with a switch in monocyte phenotype, we also found a switch in monocyte function in that ex vivo pro-inflammatory cytokine production increased during hypoglycaemia, whereas ex vivo anti-inflammatory production decreased (16). Somewhat surprisingly, we observed a trend towards increased rather than decreased levels of cytokine production after antecedent hypoglycaemia following ex vivo stimulation with LPS, again arguing against an attenuating effect of antecedent hypoglycaemia on the immune system.
Interestingly, we found an attenuating effect of antecedent hypoglycaemia on the hs-CRP response to next-day hypoglycaemia. However, this attenuated response was not directly related to the suppressed adrenaline response in participants exposed to prior hypoglycaemia. Therefore, it could be that adrenaline is not the origin of the pro-inflammatory response but a marker of the response.
When looking at other circulating markers of inflammation, we found a sustained increase lasting up to one week of various circulating inflammatory proteins, including IL-12B. This cytokine stimulates T and NK-cells to produce IFN-γ, which was also found to increase in response to hypoglycaemia (17). IFN-γ in turn activates macrophages, which then produce TNF-α (18). In addition, CXCL9, CCL23, CCL3 and CLL4 increased in response to hypoglycaemia, which can recruit immune cells, thus potentially contributing to the persistently elevated levels of circulating white blood cells (19). Overall, these observations indicate that hypoglycaemia activates both the innate and adaptive immune system and that the activation lasts for one week. Interestingly, we found fewer proteins to be elevated in the HYPO group, which suggests that antecedent hypoglycaemia may have a modest inhibiting effect on the immune activation following hypoglycaemia. This has also been seen in animal models where in both mice with and without type 1 diabetes, recurrent hypoglycaemic amplified inflammatory markers in hippocampal homogenates (20).
The adrenaline response has been suggested to be crucial in starting the inflammatory response following hypoglycaemia. Exposure to adrenaline has been associated with inflammatory changes in granulocytes and monocytes in vitro and in vivo (21). In agreement with previous observations (22), we observed a diminished adrenaline response in the HYPO group compared to the NORMO group. We found a positive correlation between the adrenaline response during hypoglycaemia and increase in lymphocyte numbers in the total study population and a similar trend for numbers of granulocytes and monocytes. The increase in pro-inflammatory cytokines following hypoglycaemia was independent of the adrenaline response and the exposure to antecedent hypoglycaemia. This contrasts with previous research that found an immunosuppressive effect of adrenaline on ex vivo cytokine production (21). Overall, our findings suggests that the pro-inflammatory effects of hypoglycaemia, as investigated here, are not solely driven by the adrenaline response. It is also possible that the adrenaline response that we observed in the antecedent hypoglycaemia group, albeit attenuated, was still sufficient in initiating the immune response. Other hormones like cortisol can be involved in the pro-inflammatory response (3). The lack of correlation between cortisol and the inflammatory readouts argues against this option.
Apart from a somewhat lower number of proteins activated by hypoglycaemia after antecedent events, overall there was largely no adaptation (at least not after two episodes) regarding the inflammatory response to hypoglycaemia. This is in contrast with the attenuating effect of recurrent hypoglycaemia on hormone responses to subsequent hypoglycaemia. Previously, an attenuated inflammatory response to hypoglycaemia was found in people with impaired awareness of hypoglycaemia (4). One could speculate that this attenuated response might protect against harmful effects of subsequent hypoglycaemia. However, a blunted adrenaline response to hypoglycaemia following recurrent episodes of hypoglycaemia did not protect against markers of vascular dysfunction (2). Furthermore, persistent prothrombotic effects have been reported following recurrent hypoglycaemia (13). These findings, along with ours, suggest that the inflammatory response following hypoglycaemia occurred largely irrespective of prior exposure to hypoglycaemia, level of hypoglycaemic awareness and glycaemic control.
Our study has limitations. The induced hypoglycaemia events were highly controlled and maintained for a certain duration with the hyperinsulinaemic glucose clamp technique in people without diabetes, whereby the hypoglycaemic events may differ in depth, duration and number from spontaneous hypoglycaemia in daily life in people with diabetes treated with insulin. However, this method ensured that all participants underwent an identical hypoglycaemic event, so that we were able to compare the results between the two groups. Also, we cannot exclude an attenuating effect of more than two hypoglycaemic events on the inflammatory response, although this still does not explain the discrepancy with adrenaline. Another limitation is that the rather high doses of insulin, such as those used for the clamps, can reduce pro-inflammatory responses (2). However, despite any potential anti-inflammatory effects of insulin, we were still able to observe pro-inflammatory effects of hypoglycaemia.
Our study has also strengths. The two investigated groups were well matched for age, sex and BMI. In addition, we provided a comprehensive assessment of the inflammatory profile, including leukocyte cell counts, phenotype of monocytes, function of monocytes and circulating inflammatory proteins. Finally, unlike most previous studies on the consequences of hypoglycaemia on inflammation, our investigations continued up to a week after the hypoglycaemic event.
In conclusion, our study provides evidence that the inflammatory response to hypoglycaemia occurs largely independent of prior exposure to hypoglycaemia and the release of adrenaline. Future research is needed to elaborate further on the mechanisms that underly the observed inflammatory effect, and the potential role of the inflammatory response on the development of cardiovascular complications.