The primary influenza A virus-specific CD8؉ -T-cell responses measured by tetramer staining of spleen, lymph node, and bronchoalveolar lavage (BAL) lymphocyte populations were similar in magnitude for conventional I-A b؉/؉ and CD4 ؉ -T-cell-deficient I-A b؊/؊ mice. Comparable levels of virus-specific cytotoxic-Tlymphocyte activity were detected in the inflammatory exudate recovered by BAL following challenge. However, both the size of the memory T-cell pool and the magnitude of the recall response in the lymphoid tissues (but not the BAL specimens) were significantly diminished in mice lacking the CD4 ؉ subset. Also, the rate of virus elimination from the infected respiratory tract slowed at low virus loads following challenge of naïve and previously immunized I-A b؊/؊ mice. Thus, though the capacity to mediate the CD8 ؉ -T-cell effector function is broadly preserved in the absence of concurrent CD4؉ -T-cell help, both the maintenance and recall of memory are compromised and the clearance of residual virus is delayed. These findings are consistent with mathematical models that predict virus-host dynamics in this, and other, models of infection.

The spectrum of TCR V beta usage is compared for primary and recall CD8(+)D(b)PA(224)(+) T cell responses in mice with influenza pneumonia. Single-cell RT-PCR established that the same clonotypes were present in the lymphoid tissue and in the virus-infected lung. Longitudinal analysis indicated that the memory TCR repertoire reflects the primary response, with no decrease in diversity prior to (or after) secondary challenge. The re-engagement of memory T cells looked to be stochastic in this localized, transient infection. Analysis of clonotypes from the blood, spleen, regional lymph nodes, bone marrow, lung, and liver over a 200 day interval showed no evidence of selective localization or loss. The long-term distribution of memory T cells seemed to be essentially random.

Conventional memory CD8+ T cells and mucosal-associated invariant T cells (MAIT cells) are found in blood, liver, and mucosal tissues and have similar effector potential following activation, specifically expression of IFN-γ and granzyme B. To better understand each subset’s unique contributions to immunity and pathology, we interrogated inflammation- and TCR-driven activation requirements using human memory CD8+ T and MAIT cells isolated from blood and mucosal tissue biopsies in ex vivo functional assays and single cell gene expression experiments. We found that MAIT cells had a robust IFN-γ and granzyme B response to inflammatory signals but limited responsiveness when stimulated directly via their TCR. Importantly, this is not due to an overall hyporesponsiveness to TCR signals. When delivered together, TCR and inflammatory signals synergize to elicit potent effector function in MAIT cells. This unique control of effector function allows MAIT cells to respond to the same TCR signal in a dichotomous and situation-specific manner. We propose that this could serve to prevent responses to antigen in noninflamed healthy mucosal tissue, while maintaining responsiveness and great sensitivity to inflammation-eliciting infections. We discuss the implications of these findings in context of inflammation-inducing damage to tissues such as BM transplant conditioning or HIV infection.

Recent studies indicate that IFN-γ may influence both the expansion and the trafficking of virus-specific CD8+ CTL, though the effects are not necessarily consistent for different models of viral and bacterial disease. Influenza A virus infection of mice deficient for IFN-γ (IFN-γ−/−) or deficient for the IFN-γ receptor 1 (IFNGR1−/−) was, when compared with the wild-type (WT) B6 controls, associated with increased Ag-specific CD8+ T cell counts in the spleen and mediastinal lymph nodes. At the same time, fewer of these CTL effectors were found in the bronchoalveolar lavage population recovered from the IFN-γ−/− mice. Comparable effects were observed for WT mice treated with a neutralizing IFN-γ-specific mAb. Transfer of WT memory Thy1.1+ CD8+ populations into Thy1.2+ B6 IFN-γ−/− or IFNGR1−/− mice followed by intranasal virus challenge demonstrated both that IFN-γ produced by the host was important for the regulation of Ag-specific CTL numbers and that IFN-γ was likely to act directly on the T cells themselves. In addition, the prevalence of CTLs undergoing apoptosis in spleen was lower when measured directly ex vivo for IFN-γ−/− vs WT B6 mice. The present analysis is the first comprehensive demonstration that IFN-γ signaling can differentially regulate both Ag-specific CTL homeostasis in secondary lymphoid organs and trafficking to a site of virus-induced pathology.

Background Prevention of rectal HIV transmission is a high priority goal for vaccines and topical microbicides because a large fraction of HIV transmissions occurs rectally. Yet, little is known about the specific target cell milieu in the human rectum other than inferences made from the colon. Methods We conducted a comprehensive comparative in situ fluorescence study of HIV target cells (CCR5-expressing T cells, macrophages and putative dendritic cells) at 4 and 30 cm proximal of the anal canal in 29 healthy individuals, using computerized analysis of digitized combination stains. Results Most strikingly, we find that more than three times as many CD68+ macrophages express the HIV co-receptor CCR5 in the rectum than the colon (p=0.0001), and as such rectal macrophages appear biologically closer to the HIV-susceptible CCR5high phenotype in the vagina than the mostly HIV-resistant CCR5low phenotype in the colon. Putative CD209+ dendritic cells are generally enriched in the colon compared to the rectum (p=0.0004), though their CCR5 expression levels are similar in both compartments. CD3+ T cell densities and CCR5 expression levels are comparable in the colon and rectum. Conclusions Our study establishes the target cell environment for HIV infection in the human distal gut and demonstrates in general terms that the colon and rectum are immunologically distinct anatomical compartments. Greater expression of CCR5 on rectal macrophages suggests that the most distal sections of the gut may be especially vulnerable to HIV infection. Our findings also emphasize that caution should be exercised when extrapolating data obtained from colon tissues to the rectum.