Lisheng Lü

CD4(+) recent thymic emigrants (RTEs) comprise a clinically and immunologically important T cell population that indicates thymic output and that is essential for maintaining a diverse alphabeta-T cell receptor (TCR) repertoire of the naive CD4(+) T cell compartment. However, their frequency and function are poorly understood because no known surface markers distinguish them from older non-RTE naive CD4(+) T cells. We demonstrate that protein tyrosine kinase 7 (PTK7) is a novel marker for human CD4(+) RTEs. Consistent with their recent thymic origin, human PTK7(+) RTEs contained higher levels of signal joint TCR gene excision circles and were more responsive to interleukin (IL)-7 compared with PTK7(-) naive CD4(+) T cells, and rapidly decreased after complete thymectomy. Importantly, CD4(+) RTEs proliferated less and produced less IL-2 and interferon-gamma than PTK7(-) naive CD4(+) T cells after alphabeta-TCR/CD3 and CD28 engagement. This immaturity in CD4(+) RTE effector function may contribute to the reduced CD4(+) T cell immunity observed in contexts in which CD4(+) RTEs predominate, such as in the fetus and neonate or after immune reconstitution. The ability to identify viable CD4(+) RTEs by PTK7 staining should be useful for monitoring thymic output in both healthy individuals and in patients with genetic or acquired CD4(+) T cell immunodeficiencies.

3061 Background: Interaction of immune checkpoint molecules PD-1 and CTLA-4 and their respective ligands attenuates antitumor T cell responses. In clinical studies, PD-1 blocking antibody (Ab) nivolumab (BMS-936558) or the CTLA-4 blocking Ab ipilimumab result in durable responses in multiple human malignancies. We describe the evaluation of concurrent treatment with anti-PD-1 and anti-CTLA-4 mAbs in preclinical models. Methods: Antitumor activity of treatment with murine homologs of anti-PD-1 (4H2-mIgG1) and anti-CTLA-4 (9D9-mIgG2b) was evaluated in MC38, a murine colon adenocarcinoma model. The effects of concurrent treatment on T cell infiltration of tumors, tumoral expression of PD-L1 and cytokine levels were explored. The preclinical safety profile of concurrent nivolumab + ipilimumab was assessed in a cynomolgus macaque model. Results: Concurrent treatment of MC38 tumors with 4H2-mIgG1 + 9D9-mIgG2b (10 mg/kg Q3d x 3) results in synergistic antitumor activity whereas efficacy with sequential dosing was similar to either agent alone. With concurrent treatment, dose reductions of one Ab relative to a fixed dose of the other resulted in retention of some antitumor activity. Anti-PD-1 enhanced CD8+ T cell infiltration of MC38 tumors and increased tumor PD-L1 expression. Anti-CTLA-4 treatment increased intratumoral CD8+ T cells and reduced intratumoral T regulatory cells. While concurrent treatment did not result in further increases in T cell infiltration, it increased expression of intratumoral cytokines. Anti-PD-1 resulted in down regulation of cell surface and intracellular levels of PD-1 in CD8+ T cells. In cynomolgus macaques, concurrent nivolumab + ipilimumab resulted in dose-dependent gastrointestinal toxicities (diarrhea; body weight loss) not observed in earlier cynomolgus studies with nivolumab and rarely with ipilimumab. These preclinical observations provided the rationale for a dose escalation trial (NCT01024231) of combined nivolumab + ipilimumab in advanced melanoma. Conclusions: Concurrent treatment with anti-PD-1/anti-CTLA-4 resulted in synergistic antitumor activity in preclinical models and supports the evaluation of the combination in clinical studies.

The yolk sac is the first site of hematopoiesis during mammalian development. The yolk sac is also the first site of blood vessel development. Development of the blood islands in the yolk sac is an integrated process in which these two developmental events, hematopoiesis and vasculogenesis, proceed in concert. This review focuses on mouse yolk sac hematopoietic stem cells (YS-HSC), describing their differentiation in vitro and in vivo. YS-HSC go through a progressive series of changes prior to the initiation of lineage-specific differentiation. Experiments tracing their origins from postulated hemangioblasts, and the subsequent interaction between these stem cells and yolk sac endothelial cells are described. Differences between the extraembryonic YS-HSC and HSC found later within the embryo, perinatally or in adults, are described. YS-HSC have greater reproductive capability than HSC obtained from fetal liver, umbilical cord blood or adult bone marrow; they do not yet express major histocompatibility complex-associated antigens and they are able to reconstitute adult immunocompromised animals even when introduced in small numbers (< 100 cells/mouse). With recent results demonstrating the feasibility of expanding YS-HSC in vitro as well as of introducing new genes into these cells by transfection, the YS-HSC shows promise both as a means of achieving long-term restitution of hematopoiesis across histocompatibility barriers and as a self-renewing vehicle for gene transfer.