The CD28/B7 costimulatory pathway is generally considered dispensable for memory T cell responses, largely based on in vitro studies demonstrating memory T cell activation in the absence of CD28 engagement by B7 ligands. However, the susceptibility of memory CD4 T cells, including central (CD62Lhigh) and effector memory (TEM; CD62Llow) subsets, to inhibition of CD28-derived costimulation has not been closely examined. In this study, we demonstrate that inhibition of CD28/B7 costimulation with the B7-binding fusion molecule CTLA4Ig has profound and specific effects on secondary responses mediated by memory CD4 T cells generated by priming with Ag or infection with influenza virus. In vitro, CTLA4Ig substantially inhibits IL-2, but not IFN-γ production from heterogeneous memory CD4 T cells specific for influenza hemagglutinin or OVA in response to peptide challenge. Moreover, IL-2 production from polyclonal influenza-specific memory CD4 T cells in response to virus challenge was completely abrogated by CTLA4Ig with IFN-γ production partially inhibited. When administered in vivo, CTLA4Ig significantly blocks Ag-driven memory CD4 T cell proliferation and expansion, without affecting early recall and activation. Importantly, CTLA4Ig treatment in vivo induced a striking shift in the phenotype of the responding population from predominantly TEM in control-treated mice to predominantly central memory T cells in CTLA4Ig-treated mice, suggesting biased effects of CTLA4Ig on TEM responses. Our results identify a novel role for CD28/B7 as a regulator of memory T cell responses, and have important clinical implications for using CTLA4Ig to abrogate the pathologic consequences of TEM cells in autoimmunity and chronic disease.

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Mechanisms for the generation of memory CD4 T cells and their delineation into diverse subsets remain largely unknown. In this study, we demonstrate in two Ag systems, divergent generation of heterogeneous memory CD4 T cells from activated precursors in distinct differentiation stages. Specifically, we show that influenza hemagglutinin- and OVA-specific CD4 T cells activated for 1, 2, and 3 days, respectively, exhibit gradations of differentiation by cell surface phenotype, IFN-γ production, and proliferation, yet all serve as direct precursors for functional memory CD4 T cells when transferred in vivo into Ag-free mouse hosts. Using a conversion assay to track the immediate fate of activated precursors in vivo, we show that day 1- to 3-activated cells all rapidly convert from an activated phenotype (CD25highIL-7RlowCD44high) to a resting memory phenotype (IL-7RhighCD25lowCD44high) 1 day after antigenic withdrawal. Paradoxically, stable memory subset delineation from undifferentiated (day 1- to 2-activated) precursors was predominantly an effector memory (CD62Llow) profile, with an increased proportion of central memory (CD62Lhigh) T cells arising from more differentiated (day 3-activated) precursors. Our findings support a divergent model for generation of memory CD4 T cells directly from activated precursors in multiple differentiation states, with subset heterogeneity maximized by increased activation and differentiation during priming.

Optimization of long term immunity depends on the functional persistence of memory T cells; however, there are no defined strategies for promoting memory T cell function and survival. In this study, we hypothesized that varying the strength of the recall stimulus could modulate the function and survival potential of memory CD4 T cells. We tested the ability of peptide variants of influenza hemagglutinin (HA) exhibiting strong and weak avidity for an HA-specific TCR, to modulate HA-specific memory CD4 T cells in vitro and in vivo. In vitro stimulation with a weak avidity peptide (L115) uncoupled memory CD4 T proliferation from effector cytokine production with low apoptosis, whereas stimulation with a strong avidity peptide (Y117) fully recalled memory T cell functions but triggered increased apoptosis. To determine how differential recall would affect memory T cells in vivo, we boosted BALB/c hosts of transferred, CFSE-labeled HA-specific memory CD4 T cells with native HA, Y117, and L115 variant peptides and found differences in early Ag-driven memory T cell proliferation and IL-7R expression, with subsequent changes in memory T cell yield. High avidity boosting resulted in rapid proliferation, extensive IL-7R down-regulation, and the lowest yield of HA-specific memory cells, whereas low avidity boosting triggered low in vivo proliferation, maintenance of IL-7R expression, and the highest memory T cell yield. Our results indicate that memory CD4 T cell function and survival can be modulated at the recall level, and can be optimized by low level stimulation that minimizes apoptosis and enhances responses to survival factors.

In this relatively low HIV prevalence population of pregnant women in India, the sensitivity of the rapid HIV tests varied, when compared to a dual EIA algorithm. In general, the specificity of all the rapid tests was excellent, with very few false positive HIV tests. Based upon these data, two different rapid HIV tests for screening pregnant women in India would be highly sensitive, with excellent specificity to reliably prevent inappropriate use of antiretroviral therapy for prevention of vertical HIV transmission.