In the present study, with the use of pharmacologic depletion of circulating B cells,
JHD
−/−
mice, and AT experiments of B cells into
Rag1
−/−
mice, we independently confirm that B cells play a minor role in acute IS. Pharmacologic and genetic depletion of B cells did not affect stroke size in all of the experimental approaches [
3,
7,
8]. Also, transferring B cells into
Rag1
−/−
mice did not neutralize the neuroprotective effect of immune deficiency although there was a slight increase of infarct volumes after AT of B cells what is consistent with previous studies [
7]. Importantly,
Rag1
−/−
mice supplemented with T cells were fully susceptible to ischemic brain damage, proving that T cells are detrimental in early stroke development.
Moreover, we did not find evidence that B cells contribute to the local inflammatory response because ipsilesional numbers of monocytes and neutrophils, cells that are well known to appear within the first 3 days after tMCAO [
2], as well as expression levels of the pro-inflammatory cytokines TNFα and IL1β, were comparable in mice lacking B cells and corresponding WT controls. Consequently, neuronal apoptosis and astrocyte reactivity that seem to correlate with the degree of immune activity [
2] were also indistinguishable in WT and B cell-deficient mice. In contrast to previous studies [
3,
7,
8] and our results (all arguing for a negligible role of B cells during the acute phase of cerebral ischemia), it has been reported that B cells are cerebroprotective immunomodulators through anti-inflammatory effects [
4,
6]. The reasons for the differences between these studies may partially be explained by the use of different experimental settings. While the Offner group [
4,
6] used
μMT
−/−
mice as a B cell-deficient mouse strain, we used the
JHD
−/−
mice, and also, the number of B cells used for adoptive transfer are different (5 million vs 750.000). Moreover, the extent of brain damage (large infarctions vs moderate infarctions) might have differentially influenced the function of B cells and, therefore, stroke outcome [
17].These discrepant findings clearly underline that research on the impact of B cells in stroke lies in its infancy, and it is still incompletely understood how the immune system is regulated and contributes to the pathophysiology of an ischemic insult. Besides the strengths of our investigation (using three independent approaches to investigate the influence of B cells in the acute phase of cerebral ischemia), additional studies are needed to finally evaluate the relevance of B cells in IS. B cell intervention should be studied in different stroke models as it is known that there could be differences in transient versus permanent stroke models without reperfusion. Furthermore, distinct time lines of B cell interventions should be investigated. In this study, AT of B cells was performed 1 day prior to experimental stroke and we cannot rule out that other time lines might cause other findings. For a better translational relevance, B cell intervention should also be addressed after the ischemic stimulus. In addition, the role of B cells in the later phase of post stroke recovery requires further investigations. To overcome the poor reproducibility of preclinical trials and poor translation to the clinic, multicenter preclinical randomized controlled trials (pRCTs) have been proposed as a suitable tool for “bridging the gap” between experimental research and clinical trials [
18]. In such pRCTs, standardized protocols in collaboration with independent multinational research centers are used and such an approach appears best suitable to address such a challenging task.
The present study indicates that B cells play a minor role in infarct development during acute IS.