Cytokine relay from the peripheral to the central: Secrets behind fever

Fever is often triggered by infections or inflammatory conditions, primarily mediated by the immune system. Immune cells like macrophages and dendritic cells detect pathogens through pathogen-associated molecular patterns, such as lipopolysaccharides (LPS).¹ In response, these immune cells release a significant number of inflammatory factors or cytokines, which travel through the bloodstream to the hypothalamus, the body's thermoregulatory center. Once in the hypothalamus, these cytokines stimulate various cells, including microglia—the innate immune cell in the central nervous system.² This stimulation initiates a complex cascade that raises body temperature. However, the precise mechanisms by which hypothalamic microglia interact with peripheral immune cells to induce fever remain unclear.³

In this issue of Acta Physiologica, Yu et al. elucidate the molecular mechanisms of fever driven by interactions between peripheral macrophages and preoptic anterior hypothalamus (POAH) microglia.⁴ In this study, they administered 20 μg/kg of LPS via the tail vein, which triggered a characteristic biphasic fever at 2 and 6 hours post-injection (hpi). At each time point, the levels of key pro-inflammatory cytokines involved in fever development, including IL-1β, IL-18, interferon (IFN)-β, and TNF-α, were measured. At 2 hpi, there was a slight but not significant increase in the number of macrophages in the blood and in the levels of cytokines in the serum. However, by 6 hpi, there was a significant increase in both peripheral macrophages and CNS microglia, accompanied by a dramatic rise in PGE2 and IL-1β levels in the blood and POAH region. Importantly, this was not due to LPS entering the brain, as neither Evans blue nor FITC-LPS applied peripherally was detected in the brain, indicating that microglia activation was not a direct result of LPS exposure. As these sets of cytokines are mainly expressed by macrophages and microglia, authors hypothesized that the activation of microglia is due to the entry of cytokines derived from macrophages.

To further investigate, the authors selectively depleted peripheral macrophages by administering clodronate-liposome via tail-vein injection 24 h before LPS treatment. In the absence of macrophages, even after 6 h of LPS injection, neither the microglia number nor the body temperature changed. Depleting macrophages also suppressed the LPS-induced increase in cytokine levels in both serum and the POAH region, suggesting that peripheral macrophages play a key role in fever development. Conversely, when POAH microglia were depleted using the same drug injected directly into the POAH region, cytokine levels in the serum were similarly elevated but remained low in the PO/AH region even after LPS treatment. Although body temperature was significantly reduced in comparison to LPS-treated control mice, it remained slightly higher than in healthy mice, likely due to the remaining action of macrophage-derived cytokines in POAH. Using bulk RNA-sequencing, the authors identified a significant increase in genes associated with the NOD-like receptor signaling pathway, particularly the Caspase11-NLRP3 inflammasome, in the POAH region 6 hpi of LPS, especially in microglia. These observations suggest that the first phase of fever (2 hpi LPS injection) may be attributed to the direct effect of macrophage-derived cytokines entering the brain, while the second temperature peak is driven by cytokine amplification within hypothalamic microglia. These data support the hypothesis of a cytokine relay and amplification mechanism between peripheral macrophages and central microglia in fever development.

In vitro co-culture experiments revealed that both the LPS-treated activated bone marrow-derived macrophages (BMDM) and BV-2 microglia co-cultured with activated BMDM upregulate cytokines. In addition, these microglia significantly increased the Caspase11 expression. This effect was also observed when BV-2 microglia were treated with a conditioned medium from activated BMDM cells and was further amplified when the co-culture system was incubated at 39°C, mimicking high body temperature. This suggests that microglial Caspase11 is involved in the cytokine relay between macrophages and microglia, contributing to fever development. By selectively silencing microglia-specific Caspase11, the authors were able to suppress the second temperature peak without affecting the first, while cytokine expression in the POAH region returned to healthy levels. Conversely, overexpressing Caspase11 in POAH microglia significantly increased cytokine levels in this region, and the second phase of temperature increase was slightly elevated.

The study significantly advances our understanding of the molecular mechanisms underlying fever by identifying Caspase11 in microglia as a key player in driving fever through the non-canonical inflammasome pathway. In addition, the intricate cooperation within the body, involving a cytokine relay from the periphery to the CNS, is of great interest, as it could lead to new therapeutic approaches for managing fever in various clinical settings.

In the future, further studies are needed to determine whether macrophage-derived cytokines directly act on POAH microglia or other cell populations. Endothelial cells lining brain blood vessels also respond to cytokines and produce PGE2, further facilitating the fever response.⁵ A recent study demonstrated that LPS triggers an increase in peripheral adenosine, which acts on astrocytes and further modulates microglial reactivity in a systemically induced sepsis model.⁶ Oligodendrocyte precursor cells, some of which locate on blood vessels,⁷ can interact with immune cells^{8, 9} and participate in immune modulation.¹⁰ Therefore, it remains to be studied whether, apart from the direct action of macrophage-derived cytokines on POAH microglia, other glial cells in the CNS are involved in driving fever. It is also unclear how microglia-derived cytokines modulate fever, either directly or by interacting with other glial cells, such as oligodendrocytes.¹¹ Additionally, detailed investigations are required to clarify whether the interaction between macrophages and microglia involves direct cytokine relay or indirect mechanisms. For instance, tracing macrophage-derived cytokines (using Rosa26-methionyl tRNA synthetase reporter) in microglia would be a promising starting point.

Xianshu Bai: Writing – original draft; conceptualization.