Introduction: The sterile inflammation in consequence of cerebral ischemia is accompanied by the immigration and activation of hematogenous and brain resident immune cells. It has been shown that the nuclear receptor NR4A1 (or Nerve Growth Factor IB) exerts a crucial role within the inflammatory response via regulating macrophage activation. In this study, we aimed to investigate the influence of NR4A1 in a loss-of-function paradigm on the activation and recruitment of both, brain resident and hematogenous immune cells following experimental stroke in mice.
Methods: Wildtype (n=19) and NR4A1-deficient (n=18) mice were subjected to ischemia of the MCA (MCAO, 30 min). Animals were tested for functional outcome and were sacrificed on day 1 and 3 after reperfusion. Infarct volume, expression of inflammatory genes and activation/immigration of immune cells were analyzed via RT-PCR, (immuno-)histology and FACS analyses.
Results: Lack of NR4A1 led to worsened functional outcome (p<0.001) and increased infarct volume (p<0.01) compared to wildtype mice. NR4A1-deficient mice showed enlarged astroglial scar formation (p<0.01) and increased COX2 expression (p<0.05). Histological evaluation showed increased macrophage counts on day 3 (p<0.05) following MCAO in NR4A1-deficient mice. Flow cytometric quantification of CD11b+/CD45low-cells showed a significantly diminished amount of microglia in NR4A1-deficient mice on day 3 compared to the wildtype group (p<0.001). NR4A1-deficiency resulted in an increased immigration of Ly6G+-granulocytes on day 3 after ischemia (p<0.001). Furthermore, Ly6G+-cells of NR4A1-deficient mice showed an excessively increased expression of MHC-class-II compared to wildtype mice (p<0.001).
Conclusion: NR4A1-deficient mice developed enlarged infarcts and aggravated functional outcomes. Lack of NR4A1 led to impaired microglia activation and increased influx of Ly6G+-neutrophil granulocytes. NR4A1-deficient mice showed a strikingly increased number of MHC-II-presenting neutrophils compared to the wildtype group. Taken together, these results reveal a prominent influence of the receptor NR4A1 in regulation and activation of the cellular immune response and development of subsequent brain injury.