Apoptosis in monocytes.

ARTICLE

SPONTANEOUSLY OCCURING APOPTOSIS IN MONOCYTES INVOLVES FAS/FASL INTERACTION AND IS INFLUENCED BY CYTOKINES AND MICROBIAL INFECTIONS

It is known that peripheral antigen-activated lymphocytes become apoptotic [1], but it has been shown also that Mo cultured in vitro without any stimulus become apoptotic within less than 24 hours [2, 3]. The majority of human Mo/Mø express both, Fas and FasL [3]. Spontaneous Mo apoptosis can be reduced by antagonistic antibodies to Fas and FasL, indicating an autocrine or paracrine role for Fas/FasL interactions in Mo apoptosis. During acute inflammation, cytokines are produced by different cell types which differentially affect Mo survival. Pro-inflammatory cytokines such as tumour necrosis factor (TNF)-TNF-alpha, interleukin (IL)-1ß, and granulocyte monocyte-colony stimulating factor (GM-CSF) abrogate spontaneous apoptosis in Mo [2], although TNF-alpha also delivers a death signal in TNF-alpha-sensitive tumour cells [4]. Lipopolysaccharide (LPS), a cell wall component of Gram-negative bacteria [5], is a potent inducer of pro-inflammatory cytokines in Mo and also prevents Mo apoptosis. Cytokines and LPS differentially affect the susceptibility of Mo to Fas-mediated apoptosis [6]. Agonistic anti-Fas antibody can abrogate the TNF-alpha- or IL-1ß-mediated prevention of Mo apoptosis, whereas LPS-induced Mo survival remains unchanged. LPS, in contrast to IL-1ß, inhibits the Fas-dependent elevation of intracellular reactive oxygen radicals, which may be involved in the death process in Mo. T cell-derived cytokines such as IL-4 and IFN-gamma seem to have opposite effects on Mo survival [2]. While both cytokines alone do not markedly change Mo survival, IL-4 reverses the preventing effect of TNF-alpha, IL-1ß, and LPS, possibly resulting in a reduction of pro-inflammatory cell responses, and IFN-gamma counteracts the effect of IL-4 on Mo survival. However, none of these cytokines changes Fas or FasL expression [3, 6]. Besides the preventing effect of pro-inflammatory cytokines on Mo survival, also in vitro infection of Mo with low doses of viable Mycobacterium tuberculosis (bacteria:cell = 1:1) prevents Mo from undergoing spontaneous apoptosis [7]. These low doses of M. tuberculosis induce the release of IL-1ß, TNF-alpha [7], and GM-CSF, and neutralizing anti-TNF-alpha [7] and anti-GM-CSF antibodies abrogate the mycobacteria-induced prevention of Mo apoptosis. Interestingly, the prevention from apoptosis by mycobacteria is more prominent in Mo which are bacteria-free compared to those Mo which have ingested bacteria, indicating that mycobacteria-containing Mo produce factors which can prolong survival of non-infected Mo. These findings suggest that prevention of Mo apoptosis by mycobacteria might be partially mediated by the induction of pro-inflammatory cytokines. In contrast to infection with low doses of bacteria, extracellular bacteria added to Mo at high doses (20:1), reflecting later and uncontrolled stages of infection, induce apoptosis, as shown for Staphylococcus aureus, E. coli, Pseudomonas aeruginosa, or Salmonella enteritidis [8].

MACROPHAGES AND THEIR ROLE FOR REMOVAL OF APOPTOTIC CELLS

Mo activated by cytokines or pathogens can migrate into the inflammatory tissue and further differentiate into Mø. In vitro cultured Mo and Mø comparably express Fas, but in contrast to Mo, Mø are resistant to Fas-mediated killing via activating anti-Fas mAb [3]. Mø can recognize and engulf senescent inflammatory cells before they die and release histotoxic agents [9]. This engulfment does not trigger Mø to produce chemokines or pro-inflammatory mediators [10]. At least four classes [9] of receptors on Mø mediate the recognition of senescent cells: 1) Lectins binding to altered carbohydrates on the apoptotic cells, 2) the thrombospondin receptor (CD36) and the vitronectin receptor (TNF-alpha_Vß₃) co-operating in the binding of thrombospondin, which provides the bridge from the Mø to an unknown thrombospondin binding motif on senescent cells [10], 3) a putative receptor for phosphadidylserin on apoptotic cells, and 4) CD14, recently described to be involved in recognition and clearance of apoptotic cells [11].

MONOCYTE APOPTOSIS DURING INTERACTION WITH T CELLS

Mo are effective in presentation of bacterial or viral antigens to T cells, thereby inducing an adaptive immune response, characterized by proliferation and IFN-gamma release [12]. In turn, IFN-gamma increases the phagocytic activity and HLA-DR expression on Mo/Mø [13], leading to an enhanced antigen-presenting capacity for T cell activation. Uncontrolled expansion of these responses can be prevented by the elimination of activated Mo and or T cells via apoptosis. For example, Mo are killed by CD4 and CD45RO positive recall antigen-activated memory T cells [14]. On the other hand, Mo are essential for spontaneous and activation-induced apoptosis of T cells, in HIV patients [15]. The killing mechanism may involve interaction of Fas and Fas ligand [15], which are expressed during activation on both cell types [3, 16]. However, Mo are heterogeneous in their antigen-presenting capacity for T cells and in their susceptibility to killing by the activated T cells. A subset of Mo (< 10%) lacking the Fcgamma-receptor-I (CD64) induces higher levels of IFN-gamma in PPD-(purified protein derivative of tuberculin)-activated T cells, but are less susceptible to killing by these T cells than the majority of Mo which is CD64-positive [16]. Fas expression was comparable in both Mo subsets, but CD64 Mo show a lower IL-1ß-converting enzyme (ICE) activity. Although other cysteine proteases of the ICE/CED family are importantly involved in mediating the death signal downstream of Fas activation [17], our findings suggest that differential signalling pathways may be operative in the different Mo subsets during execution of apoptosis.

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