A role for CD36 in the regulation of dendritic cell function

Abstract

Dendritic cells (DC) are crucial for the induction of immune responses and thus an inviting target for modulation by pathogens. We have previously shown that Plasmodium falciparum-infected erythrocytes inhibit the maturation of DCs. Intact P. falciparum-infected erythrocytes can bind directly to CD36 and indirectly to CD51. It is striking that these receptors, at least in part, also mediate the phagocytosis of apoptotic cells. Here we show that antibodies against CD36 or CD51, as well as exposure to early apoptotic cells, profoundly modulate DC maturation and function in response to inflammatory signals. Although modulated DCs still secrete tumor necrosis factor-alpha, they fail to activate T cells and now secrete IL-10. We therefore propose that intact P. falciparum-infected erythrocytes and apoptotic cells engage similar pathways regulating DC function. These findings may have important consequences for the treatment of malaria and may suggest strategies for modulating pathological immune responses in autoimmune diseases.

Figure 1

Antibodies against CD36 and CD51 inhibit the LPS-induced maturation of DCs. Immature DCs (DC) were exposed to control mouse IgM, anti-CD36 (IgM), or anti-CD51 (IgG1) mAbs, control anti-CD54, or anti-MHC class I mAbs, as indicated, and left untreated or matured with LPS (+LPS). Subsequently, DCs were stained for the indicated surface molecules and analyzed by FACScan. Dead cells were excluded with PI. The MFI is indicated in each histogram. Shown is one representative experiment of six.

Figure 2

Apoptotic but not necrotic cells inhibit the maturation of viable DCs. (A) When cells are harvested for FACS analysis, live DCs can be distinguished from apoptotic DCs by forward scatter (FSC) and exclusion of PI. (B) Immature DCs (DC) were left untreated (Med), matured with LPS (LPS), or exposed to autologous apoptotic (apoptotic DC + LPS) or necrotic DCs (necrotic DC + LPS) before maturation. Subsequently, DCs were analyzed by FACScan as for Fig. 1. Shown is one representative experiment of four.

Figure 3

Apoptotic cells inhibit DC maturation in response to different stimuli. Immature DCs were exposed to medium (black bars) or to apoptotic cells (hatched bars) with or without the indicated maturation stimulus and analyzed by FACScan as for Fig. 1. The fold increase in surface marker expression was calculated from the MFI as in Table 1. All data represent the mean of three independent experiments (*, P < 0.05, paired Student's t test).

Figure 4

Modulated DCs fail to induce allogeneic T-cell responses. Proliferative allogeneic T-cell responses to DCs modulated by anti-CD36 mAb (A), by anti-CD51 mAb (B), or by apoptotic cells (C). The stimulator DCs were: immature DC alone (filled squares) or matured with LPS (open squares); DCs exposed to isotype-control Igs and then matured with LPS (open diamond); DCs exposed to anti-CD36 in (A), anti-CD51 mAb in (B) or apoptotic cells in (C) alone (filled triangle) or matured with LPS (open triangle). Shown is one representative experiment of at least three. Stimulation of T cells by modulated DCs was significantly reduced (P < 0.01) compared with mature DCs.

Figure 5

Modulated DCs fail to induce antigen-specific T-cell responses. Proliferation of the T-cell clone TB-2 to DCs modulated by anti-CD36 or anti-CD51 mAb (A, B) and pulsed with the specific polypeptide α:3–181 before maturation (A) or the specific peptide α:145–163 after maturation (B) or to DCs modulated by apoptotic cells (C) and pulsed with the specific polypeptide α:3–181 before maturation with LPS. (D, E) Secretion of IFN-γ and IL-4 by the T-cell clone TB-2 in response to DCs in medium alone (med) or modulated with isotype control Ig (iso), anti-CD36 mAb (anti-CD36), or apoptotic DCs (apoDC) before maturation with LPS with or without the specific peptide α:145–163 (pep). Shown are the mean and SD of IFN-γ and IL-4 secretion by T cells (*, P < 0.01). Symbols for A–C: open or filled symbols indicate DCs pulsed with or without antigen, respectively. DCs were exposed to isotype-control Igs (circle), anti-CD36 mAb (inverse triangle), anti-CD51 mAb (triangle), or apoptotic cells (square) and then matured with LPS. Shown is one representative experiment of at least three.

Figure 6

Effect of modulating agents on LPS-induced secretion of TNF-α, IL-12p70, and IL -10 by DCs. Cytokine secretion over 24 h by immature DCs (DC) and LPS-matured DCs alone (DC LPS) or exposed to anti-CD36 (DC anti-CD36 LPS) or isotype control (DC isoM LPS) mAb, apoptotic cells (DC apo-DC LPS), or iRBC (DC iRBC LPS). The cytokine levels in culture medium alone (medium) or supernatants from apoptotic cells alone (apo-DC) are also indicated. Shown are the mean and SD of cytokine secretion by DCs and controls exposed to mAbs (n = 13), to apoptotic cells (n = 7), or to iRBC (n = 4). ns, not significant. **, P < 0.01.