Molecular events associated with CD4-mediated Down-regulation of LFA-1-dependent adhesion

Antiviral NK cell responses in HIV infection: II. viral strategies for evasion and lessons for immunotherapy and vaccination

As is the case in other viral infections, humans respond to HIV infection by activating their NK cells. However, the virus uses several strategies to neutralize and evade the host's NK cell responses. Consequently, it is not surprising that NK cell functions become compromised in HIV-infected individuals in early stages of the infection. The compromised NK cell functions also adversely affect several aspects of the host's antiviral adaptive immune responses. Researchers have made significant progress in understanding how HIV counters NK cell responses of the host. This knowledge has opened new avenues for immunotherapy and vaccination against this infection. In the first part of this review article, we gave an overview of our current knowledge of NK cell biology and discussed how the genes encoding NK cell receptors and their ligands determine innate genetic resistance/susceptibilty of humans against HIV infections and AIDS. In this second part, we discuss NK cell responses, viral strategies to counter these responses, and finally, their implications for anti-HIV immunotherapy and vaccination.

The role of SHIP1 in T-lymphocyte life and death

SHIP1 [SH2 (Src homology 2)-containing inositol phosphatase-1], an inositol 5-phosphatase expressed in haemopoietic cells, acts by hydrolysing the 5-phosphates from PtdIns(3,4,5)P(3) and Ins(1,3,4,5)P(4), thereby negatively regulating the PI3K (phosphoinositide 3-kinase) pathway. SHIP1 plays a major role in inhibiting proliferation of myeloid cells. As a result, SHIP1(-/-) mice have an increased number of neutrophils and monocytes/macrophages due to enhanced survival and proliferation of their progenitors. Although SHIP1 contributes to PtdIns(3,4,5)P(3) metabolism in T-lymphocytes, its exact role in this cell type is much less explored. Jurkat cells have recently emerged as an interesting tool to study SHIP1 function in T-cells because they do not express SHIP1 at the protein level, thereby allowing reintroduction experiments in a relatively easy-to-use system. Data obtained from SHIP1 reintroduction have revealed that SHIP1 not only acts as a negative player in T-cell lines proliferation, but also regulates critical pathways, such as NF-kappaB (nuclear factor kappaB) activation, and also appears to remarkably inhibit T-cell apoptosis. On the other hand, experiments using primary T-cells from SHIP1(-/-) mice have highlighted a new role for SHIP1 in regulatory T-cell development, but also emphasize that this protein is not required for T-cell proliferation. In support of these results, SHIP1(-/-) mice are lymphopenic, suggesting that SHIP1 function in T-cells differs from its role in the myeloid lineage.

CD4 ligation induces activation of protein kinase C zeta and phosphoinositide-dependent-protein kinase-1, two kinases required for down-regulation of LFA-1-mediated adhesion

We previously showed that CD4 binding induced a down-regulation of LFA-1-dependent-antigen-independent adhesion of T and B lymphocytes in a phosphatidylinositol-3-kinase (PI3K)-dependent manner. We now show in A201-CD4 (+) T cell lines, that anti-CD4 Ab increases activation of phosphoinositide-dependent-protein-kinase 1 (PDK1) or PKC zeta, two main effectors down-stream from PI3K. CD4 binding also increases interactions between PI3K and activated PKCzeta and PDK1. Both events are dependent on CD4/p56Lck association, since they are not detected when p56Lck is unable to bind a truncated form of CD4 in transfected T cell lines. We also show using antisense oligonucleotides that both kinases are necessary for down-regulating LFA-1-dependent adhesion induced by CD4 signalling. We also suggest a role of PDK1 in the recruitment of the phosphatase SHP-2 in a multiprotein complex induced by anti-CD4 Ab. This study thus provides further insights into the mechanism underlying the CD4 triggered regulation of LFA-1-mediated adhesion.

p56lck, LFA-1 and PI3K but not SHP-2 interact with GM1- or GM3-enriched microdomains in a CD4-p56lck association-dependent manner

We previously showed that the association of CD4 and G(M3) ganglioside induced by CD4 ligand binding was required for the down-regulation of adhesion and that aggregation of ganglioside-enriched domains was accompanied by transient co-localization of LFA-1 (lymphocyte function-associated antigen-1), PI3K (phosphoinositide 3-kinase) and CD4. We also showed that these proteins co-localized with the G(M1) ganglioside that partially co-localized with G(M3) in these domains. In the present study, we show that CD4-p56(lck) association in CD4 signalling is required for the redistribution of p56(lck), PI3K and LFA-1 in ganglioside-enriched domains, since ganglioside aggregation and recruitment of these proteins were not observed in a T-cell line (A201) expressing the mutant form of CD4 that does not bind p56(lck). In addition, we show that although these proteins associated in different ways with G(M1) and G(M3), all of the associations were dependent on CD4-p56(lck) association. Gangliosides could associate with these proteins that differ in affinity binding and could be modified following CD4 signalling. Our results suggest that through these associations, gangliosides transiently sequestrate these proteins and consequently inhibit LFA-1-dependent adhesion. Furthermore, while structural diversity of gangliosides may allow association with distinct proteins, we show that the tyrosine phosphatase SHP-2 (Src homology 2 domain-containing protein tyrosine phosphatase 2), also required for the down-regulation of LFA-1-dependent adhesion, transiently and partially co-localized with PI3K and p56(lck) in detergent-insoluble membranes without association with G(M1) or G(M3). We propose that CD4 ligation and binding with p56(lck) and their interaction with G(M3) and/or G(M1) gangliosides induce recruitment of distinct proteins important for CD4 signalling to form a multimolecular signalling complex.

Role of integrin cross-regulation in parvovirus B19 targeting

Most viral vectors used for gene therapy lack the ability to target a defined cell population. Parvovirus B19 has a restricted tropism for human erythroid progenitor cells and uses activated alpha5beta1 integrins as coreceptors for entry [Weigel-Kelley, K.A., Yoder, M.C., and Srivastava, A. (2003). Blood 102, 3927-3933]. In this study we examined the role of coexpressed integrins in alpha5beta1 integrin coreceptor function. Antibody-mediated cross-linking of beta1, beta2, and beta3 integrins and the integrin-associated protein (IAP) increased parvovirus B19 entry into nontarget K562 cells. Functional silencing of one integrin group, however, reduced the virus uptake- promoting function of a subsequently activated integrin group, indicating that the three integrins did not operate in isolation but through shared signaling pathways. This was further corroborated by direct competition between simultaneously clustered beta2 and beta1 integrins that could be overcome by stabilizing clustered beta1 integrins in a high-affinity conformation. In contrast, parvovirus B19 entry into primary erythroid progenitor cells was characterized by strong clustering-induced beta1 integrin coreceptor activity that was not abolished by subsequent beta2 and beta3 integrin activation and was, in fact, substantially increased in the presence of preclustered beta2 and beta3 integrins. Thus, integrin function is regulated in a cell type-specific manner through coexpressed integrins and preferential parvovirus B19 entry into erythroid progenitor cells is promoted by a robust beta1 integrin response that is enhanced through stable preclustering of coexpressed integrins. These results have implications for other viral vectors that use integrins as receptors/coreceptors and for gene therapy of hematopoietic progenitor cells using parvovirus B19 vectors.

Downstream of Tyrosine Kinases-1 and Src Homology 2-Containing Inositol 5′-Phosphatase Are Required for Regulation of CD4+CD25+ T Cell Development

The adaptor protein, downstream of tyrosine kinases-1 (Dok-1), and the phosphatase SHIP are both tyrosine phosphorylated in response to T cell stimulation. However, a function for these molecules in T cell development has not been defined. To clarify the role of Dok-1 and SHIP in T cell development in vivo, we compared the T cell phenotype of wild-type, Dok-1 knockout (KO), SHIP KO, and Dok-1/SHIP double-knockout (DKO) mice. Dok-1/SHIP DKO mice were runted and had a shorter life span compared with either Dok-1 KO or SHIP KO mice. Thymocyte numbers from Dok-1/SHIP DKO mice were reduced by 90%. Surface expression of both CD25 and CD69 was elevated on freshly isolated splenic CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO, suggesting these cells were constitutively activated. However, these T cells did not proliferate or produce IL-2 after stimulation. Interestingly, the CD4(+) T cells from SHIP KO and Dok-1/SHIP DKO mice produced higher levels of TGF-beta, expressed Foxp3, and inhibited IL-2 production by CD3-stimulated CD4(+)CD25(-) T cells in vitro. These findings suggest Dok-1 and SHIP function in pathways that influence regulatory T cell development.

Transcriptional modulation of TCR, Notch and Wnt signaling pathways in SEB-anergized CD4+ T cells

Gene expression changes in CD4 + Vbeta8+ T cells energized by in vivo exposure to staphylococcal enterotoxin B (SEB) bacterial superantigen compared to CD4 + Vbeta8+ non-energic T cells were assessed using DNA microarrays containing 5184 murine complementary DNAs. Anergy in splenic T cells of SEB-immunized BALB/c mice was verified by dramatically reduced proliferative capacity and an 8 x overexpression of GRAIL mRNA in CD4 + Vbeta8+ T cells taken from mice 7 days after injection. At an Associative t-test threshold of P<0.0005, 96 genes were overexpressed or detected only in anergic T cells, while 256 genes were suppressed or not detected in anergic T cells. Six of eight differential expressions tested using real-time quantitative PCR were validated. Message for B-Raf was detected only in non-anergic cells, while expression of the TCR signaling modulator Slap (Src-like adapter protein) and the TCR zeta-chain specific phosphatase Ptpn3 was enhanced. Modulation of multiple genes suggests downregulation of Wnt/beta-catenin signaling and enhanced Notch signaling in the anergic cells. Consistent with previous reports in a non-superantigen in vivo anergy model, mRNA for CD18 and the transcription factor Satb1 (special AT-rich-binding protein 1) was increased in SEB-energized T cells. This is the first report of global transcriptional changes in CD4+ T cells made anergic by superantigen exposure.

LIME: a new membrane Raft-associated adaptor protein involved in CD4 and CD8 coreceptor signaling

Lymphocyte membrane rafts contain molecules critical for immunoreceptor signaling. Here, we report identification of a new raft-associated adaptor protein LIME (Lck-interacting molecule) expressed predominantly in T lymphocytes. LIME becomes tyrosine phosphorylated after cross-linking of the CD4 or CD8 coreceptors. Phospho-LIME associates with the Src family kinase Lck and its negative regulator, Csk. Ectopic expression of LIME in Jurkat T cells results in an increase of Csk in lipid rafts, increased phosphorylation of Lck and higher Ca²⁺ response to CD3 stimulation. Thus, LIME appears to be involved in regulation of T cell activation by coreceptors.