Maturing dendritic cells (DCs) replace unstable empty and invariant chain-associated MHC class II molecules with stable MHC class II-peptide complexes. However, the role of MHC class II rich compartments (MIIC) [(late endosomal multivesicular bodies (MVB) or lysosomal multilamellar bodies (MLB))] in Ag processing and peptide loading is not known. Potolicchio et al. (p. 4935 ) induced human monocytes to differentiate into immature DCs in vitro with GM-CSF, with or without IL-4. Cells treated with GM-CSF alone bound Abs recognizing the empty form of HLA-DR1 corresponding to 20% of total surface MHC class II molecules. Both culture conditions increased MHC class II protein expression within 12 h with maximal expression by 48 h. Approximately one-fifth of empty MHC class II surface molecules on GM-CSF-differentiated monocytes became loaded during a 4-h incubation with a specific peptide; this fraction was increased by pretreating cells with isopropanol. Most MIIC in GM-CSF-treated cells having few surface MHC class II molecules were electron dense bodies with empty molecules, whereas almost all MIIC in GM-CSF-treated cells having many surface MHC class II molecules were MVB with loaded molecules. In cells treated with GM-CSF plus IL-4, MIIC were MLB with empty molecules. Fully mature DCs had only a few empty molecules in MLB. These studies show that empty HLA-DR, formed in electron dense bodies and MLB after GM-CSF or GM-CSF/IL-4 differentiation, are loaded in MVB before arriving at the surface of fully mature DCs.

Activation of CD8+ T cells by APC-presented peptide ligands is an important step in control of intracellular pathogens. Although effector CD8+ T cells contribute to the late response to an infection, the observation that intracellular TNF-α accumulates shortly after TCR engagement suggests that activated cells also might impact the early response. Brehm et al. (p. 5043 ) detected soluble and membrane-bound TNF-α, but no IFN-γ, on CD8+ T cells after stimulation of splenocytes from naive or virus peptide-specific TCR transgenic mice with anti-CD3 mAb or viral peptide, respectively. Only activated CD44high or memory CD8+ T cells produced both TNF-α and IFN-γ. TNF-α was detected by one hour of stimulation and levels peaked after 4 h of stimulation; the cytokine was not detected in cells treated with actinomycin D or cycloheximide. Immortalized dendritic cells (DCs) pulsed with the viral peptide and cocultured with naive splenocytes from the virus peptide-specific TCR transgenic mice had increased expression of a MHC class II molecule compared with unpulsed controls. Anti-IFN-γ mAb blocked the increase, whereas an inhibitor of murine TNF-α activity further increased the expression but had no effect in the presence of the anti-IFN-γ mAb. Pulsed DCs cocultured with the peptide-specific TCR transgenic cells, or unpulsed DCs treated with supernatant from viral peptide-stimulated TCR transgenic cells, had increased MHC class II molecule expression and decreased viability. The authors show that rapid synthesis of TNF-α by stimulated CD8+ T cells impacts the adaptive immune response by altering DC maturation and decreasing DC viability.

Kovats and collaborators previously used 17β-estradiol (E2) to promote differentiation of two major subsets of dendritic cells (DCs) from mouse bone marrow progenitors in ex vivo cultures containing GM-CSF. In a follow-up to that study, the same laboratory (Mao et al., p. 5146 ) functionally characterized those CD11bhighLy6C+ and CD11bintLy6C DC subsets. CD11bintLy6C DCs appeared only in cultures containing E2. As measured by RT-PCR analysis, both subsets contained estrogen receptor α mRNA, but the CD11bhigh Ly6C+ DCs also expressed estrogen receptor β mRNA. Relative expression of MHC (class I or II) molecules, several costimulatory molecules and chemokine receptors, as determined by four-parameter flow cytometry, differed between the two subsets; increased expression of MHC class I and II molecules and CD80, CD83, and CD86 was greater on CD11bintLy6C DCs following LPS activation. However, CD11bhighLy6C+ DCs cells internalized more FITC-labeled molecules during culture with E2. Only CD11bintLy6C DCs expressed langerin mRNA and possessed Birbeck granules, which are cytoplasmic structures detected by electron microscopy in Langerhans cells. The authors conclude that E2 promotes differentiation of Langerhans type epidermal DCs.

Although NKT cells with an invariant Vα24-Jα18 rearrangement (Vα24+i NKT) can promote Th2 differentiation of T cells during allergic inflammation, the mechanism is unknown. Sen et al. (p. 4914 ) found higher surface and intracellular expression of CCR9 in Vα24+i NKT cells from patients with allergic asthma than from normal controls; CCL25, the CCR9 ligand, induced chemotaxis only of asthmatic Vα24+i NKT cells. More cells positive for CCR9+ and Vα24+i were detected immunohistochemically in the submucosa of bronchus biopsies from allergic asthma patients vs healthy subjects. IL-4, IL-13, IL-10, TGF-1β, IL-2, IFN-γ, CCL25, and CXCL9 mRNAs were highly up-regulated only in asthmatic bronchus mucosa, and stimulation with α-galactosylceramide further increased expression of IL-2, IL-10, and IFN-γ. Asthmatic, but not normal, Vα24+i NKT cells were Th1 biased and induced production of IL-4 and IL-13 by syngeneic CD3+ T cells in a contact-dependent manner. Asthmatic Vα24+i NKT cells had highest expression of adhesion molecule CD226, which was phosphorylated after treatment of those cells with CCL25 or immobilized anti-CD226 mAb. Treatment of asthmatic Vα24+i NKT cells with short hairpin CD226 RNA or anti-CD226 mAb blocked their CCL25-stimulated ability to induce CD3+ T cells to produce Th2 cytokines. Normal Vα24+i NKT cells transfected with CCR9 cDNA expressed CCR9 and CD226 at levels comparable to cells from asthma patients; CCR9 and CD226 expression levels were low in Vα24+i NKT cells from asthma patients in remission or treated with corticosteroid. Only the transfected and asthmatic cells stimulated Th2 cytokine production by CD3+ T cells. The data demonstrate that allergic asthma is induced by CCR9+CD226+Vα24+i NKT cell promotion of Th2 cytokine production by CD3+ T cells.

Lectin-binding N-linked sugar chains (glycans) containing a carboxylate residue are components of receptors on endothelial cells and macrophages. An anti-carboxylated glycan mAb from the Freeze laboratory blocks acute peritoneal inflammation in mice, suggesting that carboxylated glycans are involved in pathogenesis of inflammatory bowel diseases. In a continuation of those studies, Srikrishna et al. (p. 5412 ) found that anti-carboxylated glycan mAb treatment prevented colitis in a transfer model of Rag1−/− mice transplanted with CD4+CD45RBhigh T cells; untreated or control Ab-treated transplanted animals developed severe colitis. Colitis was prevented if mAb treatment was initiated 10 days, but not 21 days, after cell transfer. Accumulation of CD4+ T cells and macrophages and expression of mucosal addressin cell adhesion molecule-1 in the lamina propria of the colon at 7 wk after T cell transfer were reduced by mAb treatment. Colonic levels of IFN-γ and TNF-α in the mAb-treated mice were comparable to those seen in normal mice but were higher in untreated controls. The mAb reduced in vitro production of TNF-α, IL-23, IL-12, and NO from LPS-stimulated macrophages expressing the mAb-reactive epitope. Anti-carboxylated glycan mAb-treated mice did not have the up-regulation of two glycan-binding S100 proteins, their carboxylated glycan-containing receptor, and NF-κB p65 seen at 3 wk post-cell transfer in colonic lamina propria of untreated and control Ab-treated mice. This report identifies NF-κB activation as an important step in mouse colitis pathology following carboxylated glycan/lectin interaction.

Developing autoreactive B cells are inactivated through receptor editing. This mechanism, coupled with allelic exclusion, usually ensures that a lymphocyte expresses a unique BCR. However, Liu et al. (p. 5067 ) studied the development of B cells coexpressing two BCRs, reactive and nonreactive with membrane-bound H-2Kb, in 3-83Ig knockin (Igi), H-2b mice. Mice developed 3-83IgM autoantibodies by 7 mo of age, although B cells expressing surface 3-83Ig receptors were never detected. <4% of splenic IgM+ B cells in lethally irradiated H-2b mice, adoptively transferred with homozygous or hemizygous 3-83Igi, H-2d bone marrow cells, expressed surface 3-83Ig ex vivo compared with 22% after 2 days of culture in vitro. Anti-3-83IgM Ab was produced by 60 and 43% of B cell hybridomas established from LPS-stimulated spleen cells from 3-83Igi mice, homozygous or hemizygous, respectively, for H-2b. Receptor editing did not occur in B cells in 3-83Igi mice bred onto a Rag1−/−, H-2b background, resulting in no transitional 1, transitional 2, and mature splenic B cells. In the absence of H-2Kb, B cells from H-2d mice with a second nonautoreactive Igh, in addition to 3-83Igh and Igk, coexpressed BCRs with each Igh paired with 3-83Igk. Approximately 45% of B cells from H-2b mice adoptively transferred with H-2d bone marrow cells carrying 3-83Igi plus the second Igh re-expressed 3-83Ig surface Ab in vitro, compared with 0% of B cells from H-2b mice carrying 3-83Igi plus the second Igh. Surface expression of 3-83Ig on Igh/3-83Igi, H-2b spleen B cells was higher in the presence of a Src family kinase inhibitor, with or without a lysosome inhibitor, during in vitro culturing in the presence of the autoantigen. The data suggest that high avidity autoreactive B cells that escape receptor editing by expressing nonautoreactive Abs can differentiate and survive central tolerance.

Human embryonic stem cells (hESCs) can differentiate into myeloid, erythroid, and megakaryocytic cells. However, the potential of hESCs to differentiate into cells of lymphoid lineage has not been demonstrated. Woll et al. (p. 5095 ) used a two-step in vitro differentiation scheme in which a hESC cell line was cocultured first with a murine bone marrow-derived stromal cell line. Myeloid colony-forming cells, enriched by sorting for either CD34+ or CD34+ CD45+ cells, were put into a secondary culture with a murine fetal liver-derived stromal cell line under NK cell conditions (IL-15, IL-3, IL-7, stem cell factor, and flt3 ligand). Both colony-forming cell populations, as well as control umbilical cord blood CD34+ cells, expanded significantly, and 15% of the cells expressed CD56 after 21 days. Time-dependent up-regulated expression was noted for other surface molecules. CD56+ NK cells derived from hESCs lysed human erythroleukemia cells in a 51Cr release assay after 32 days in culture and mediated Ab-dependent cell-mediated cytotoxicity of NK-resistant human B lymphoma cells in vitro. Up-regulated expression of IFN-γ was seen after stimulation of hESC-derived NK cells with IL-12 plus IL-18. The authors show that they can derive functional lymphoid cells from undifferentiated hESCs and can selectively expand and induce maturation of NK cells.

Summaries written by Dorothy L. Buchhagen, Ph.D.