Different bacterial cargo in apoptotic cells drive distinct macrophage phenotypes

The removal of dead cells (efferocytosis) contributes to the resolution of the infection and preservation of the tissue. Depending on the environment milieu, macrophages may show inflammatory (M1) or anti-inflammatory (M2) phenotypes. Inflammatory leukocytes are recruited during infection, followed by the accumulation of infected and non-infected apoptotic cells (AC). Efferocytosis of non-infected AC promotes TGF-β, IL-10, and PGE2 production and the polarization of anti-inflammatory macrophages. These M2 macrophages acquire an efficient ability to remove apoptotic cells that are involved in tissue repair and resolution of inflammation. On the other hand, the impact of efferocytosis of infected apoptotic cells on macrophage activation profile remains unknown. Here, we are showing that the efferocytosis of gram-positive Streptococcus pneumoniae-AC (Sp-AC) or gram-negative Klebsiella pneumoniae-AC (Kp-AC) promotes distinct gene expression and cytokine signature in macrophages. Whereas the efferocytosis of Kp-AC triggered a predominant M1 phenotype in vitro and in vivo, the efferocytosis of Sp-AC promoted a mixed M1/M2 activation in vitro and in vivo in a model of allergic asthma. Together, these findings suggest that the nature of the pathogen and antigen load into AC may have different impacts on inducing macrophage polarization.

RIP2 Contributes to Expanded CD4+ T Cell IFN-γ Production during Efferocytosis of Streptococcus pneumoniae-Infected Apoptotic Cells

Apoptotic cell clearance by professional and nonprofessional phagocytes in the process of efferocytosis is critical to preserve tissue homeostasis. Uptake of apoptotic cells by dendritic cells generates regulatory T cells and induces immunologic tolerance against self-antigens. In contrast, ingestion of infected apoptotic cells promotes activation of TLR4/MyD88-dependent bone marrow-derived dendritic cells (BMDCs) and triggers Th17 cell differentiation. In this study, we evaluated the impact of Streptococcus pneumoniae-infected apoptotic cell efferocytosis by BMDCs derived from C57BL/6 mice on differentiation and expansion of CD4⁺ T cell subsets, as well as the role of TLR2/4 and receptor-interacting protein 2 (RIP2) receptors in recognizing intracellular pathogens during efferocytosis. We demonstrated that BMDC-mediated efferocytosis of S. pneumoniae-infected apoptotic cells induced Th1 cell differentiation and expansion. Although TLR2/4 and RIP2 deficiency in BMDCs did not affect Th1 cell differentiation during efferocytosis, the absence of RIP2 decreased IFN-γ production by CD4 T cells during the expansion phase. These findings suggest that RIP2-mediated IL-1β production during efferocytosis of S. pneumoniae-infected apoptotic cells partially supports a Th1-mediated IFN-γ production microenvironment.

Leukotriene B4 licenses inflammasome activation to enhance skin host defense

The initial production of inflammatory mediators dictates host defense as well as tissue injury. Inflammasome activation is a constituent of the inflammatory response by recognizing pathogen and host-derived products and eliciting the production of IL-1β and IL-18 in addition to inducing a type of inflammatory cell death termed "pyroptosis." Leukotriene B4 (LTB4) is a lipid mediator produced quickly (seconds to minutes) by phagocytes and induces chemotaxis, increases cytokine/chemokine production, and enhances antimicrobial effector functions. Whether LTB4 directly activates the inflammasome remains to be determined. Our data show that endogenously produced LTB4 is required for the expression of pro-IL-1β and enhances inflammasome assembly in vivo and in vitro. Furthermore, LTB4-mediated Bruton's tyrosine kinase (BTK) activation is required for inflammasome assembly in vivo as well for IL-1β-enhanced skin host defense. Together, these data unveil a new role for LTB4 in enhancing the expression and assembly of inflammasome components and suggest that while blocking LTB4 actions could be a promising therapeutic strategy to prevent inflammasome-mediated diseases, exogenous LTB4 can be used as an adjuvant to boost inflammasome-dependent host defense.

Distinctive role of efferocytosis in dendritic cell maturation and migration in sterile or infectious conditions

Efferocytosis, or clearance of apoptotic cells (ACs), by dendritic cells (DCs) leads to immune response suppression and tolerance to self-antigens. However, efferocytosis of infected apoptotic cells (IACs) leads to the production of a mixed pro- and anti-inflammatory cytokine milieu. We examined the DC phenotype and ability to migrate after phagocytosis of ACs or IACs and observed higher levels of CD86 and CCR7 expression in DCs, as well as enhanced migration capacity following efferocytosis of IACs. Interestingly, higher levels of interleukin-1β, interleukin-10 and prostaglandin E₂ (PGE₂ ) were also produced in this context. Blockage of IAC recognition led to an impaired maturation profile and PGE₂ production, which may have contributed to reduced CD86 and CCR7 expression and migration capacity. These data contribute to the understanding of how efferocytosis of sterile or infected cells may regulate the adaptive immune response, although the precise role of PGE₂ in this process requires further investigation.