Direct recognition of LPS by human but not murine CD8⁺ T cells via TLR4 complex

Abstract

LPS comprises a major PAMP and is a key target of the immune system during bacterial infection. While LPS can be recognised by innate immune cells via the TLR4 complex, it is unknown whether T lymphocytes, especially CD8 1 T cells are also capable of doing so. We report here that naïve human CD8 1 T cells, after activation by TCR stimulation, express surface TLR4 and CD14. These activated CD8 1 T cells can then secrete high concentrations of IFN-c, granzyme and perforin in response to LPS. These effects can be specifically inhibited using siRNA for TLR4. Furthermore, LPS can synergise with IL-12 to polarise the CD8 1 T cells into cytotoxic T-cell 1 (Tc1) that produce IFN-c but not IL-4, with or without TCR activation. Moreover, CD8 1 CD45RO 1 memory T cells constitutively expressed TLR4 and markedly enhanced IFN-c production when stimulated with LPS. In contrast, activated murine CD8 1 T cells lack TLR4 and CD14 expression and fail to respond to LPS for proliferation and cytokine production. Thus, human but not murine CD8 1 T cells are able to directly recognise bacterial LPS via LPS receptor complex and TLR4 provides a novel signal for the activation of effector and memory human CD8 1 T cells.Key words: CD14 . CD8 1 T cells . Tc1 . LPS . TLR Supporting Information available online IntroductionLPS is an abundant glycolipid in the outer-membrane of Gramnegative bacteria [1][2][3]. It consists of hydrophobic lipid A, the o-polysaccharide chain and a core oligosaccharide [3][4][5]. LPS is perhaps the most well-studied microbial molecule due to its powerful immune activating properties, extremely stable nature and wide distribution in the environment [1][2][3]. It is the key molecule responsible for the bacteria-triggered inflammatory response resulting in organ failure and death [1][2][3]. LPS is also closely associated with pathogenesis of many inflammatory diseases [1][2][3][4]. Despite this, the proinflammatory effects of LPS when harnessed may be beneficial; its powerful effect on both innate and adaptive immunity may provide a new generation of immune adjuvants for vaccine development against infection and cancer [2-4]. Therefore, a clearer understanding of how host recognises LPS is fundamentally important.TLR are a group of evolutionarily developed pathogen-pattern recognition receptors that are widely distributed on immune cells [3,5]. At least ten members of the TLR family in humans have been identified. These recognise different microbial products from bacteria, virus, fungi and parasites [2][3][4]. Host cells recognise LPS via a receptor complex consisting of TLR4, CD14 and MD2 in which LPS only signals through TLR4 and triggers cellular activation and expression of inflammatory genes [2][3][4]. Since TLR was originally identified in Drosophila, which lacks an adaptive immune system, the initial belief was that recognition of pathogen products was a characteristic of innate cells bearing 1564TLR [5,6]. However, the role of T cells bearing TLR in pathogen recognition is less well understood.Growing evide...

Direct recognition of LPS drive TLR4 expressing CD8+ T cell activation in patients with rheumatoid arthritis

“…Moreover, RA patients have a reservoir of auto-antigens that can act to engage TLRs and activate TLR dependent inflammation 38, 39 . Growing evidence suggests that T cells participate in immune microbial surveillance 34, 44 and our finding that the TLR4 expression correlated significantly with the severity of the disease (DAS28 scores) and that CD8 + TLR4 + T cells also secrete cytolytic molecules suggested a possible direct effect of TLR4 ligand on T cells. We thus challenged CD8 + T cells isolated from RA patients with LPS, a known ligand for TLR4.…”

“…TLR4 signaling was also proposed to enhance and sustain inflammation thereby contributing to pathogenesis of RA 43 . Recently it was also reported that human CD8 + T cells express functional LPS receptor complex and can directly recognize LPS 44 . In light of these reports we hypothesized a TLR4 mediated activation of CD8 + T cells in RA patients.…”

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“…As for the protein data, previous results have demonstrated that LPS can induce TLR4 protein expression on CD8 + T cells [41], and ODN can induce TLR9 activation on CD8 + T cells [42] as well as protein expression in other cells, such as neutrophils [40]. We hypothesize that the cigarette smoke is acting similarly by activating TLR4 and TLR9.…”

Direct TLR-2 Costimulation Unmasks the Proinflammatory Potential of Neonatal CD4+ T Cells

“…Such direct TLR-mediated co-stimulation of CD4+ T cells in the absence of APC, has been reported most consistently with TLR-2 ligands, and among adults is primarily observed in cells with a memory (CD45R0+) phenotype (11–19). The ability of the primarily naïve (20) CD4+ T cell compartment of neonates to utilize TLR to directly augment cellular immune responses in the absence of APC is unclear, and studies regarding neonatal T cell TLR expression and function are limited (12, 21, 22). …”