Naive, Effector, and Memory CD8 T Cells in Protection Against Pulmonary Influenza Virus Infection: Homing Properties Rather Than Initial Frequencies Are Crucial

The goal of adoptive immunotherapy is to target a high number of persisting effector cells to the site of a virus infection or tumor. In this study, we compared the protective value of hemagglutinin peptide-specific CD8 T cells generated from the clone-4 TCR-transgenic mice, defined by different stages of their differentiation, against lethal pulmonary influenza infection. We show that the adoptive transfer of high numbers of Ag-specific unprimed, naive CD8 T cells failed to clear the pulmonary virus titer and to promote host survival. The same numbers of in vitro generated primary Ag-specific Tc1 effector cells, producing high amounts of IFN-γ, or resting Tc1 memory cells, generated from these effectors, were protective. Highly activated CD62Llow Tc1 effectors accumulated in the lung with rapid kinetics and most efficiently reduced the pulmonary viral titer early during infection. The resting CD62Lhigh naive and memory populations first increased in cell numbers in the draining lymph nodes. Subsequently, memory cells accumulated more rapidly and to a greater extent in the lung lavage as compared with naive cells. Thus, effector cells are most effective against a localized virus infection, which correlates with their ability to rapidly distribute at the infected tissue site. The finding that similar numbers of naive Ag-specific CD8 T cells are not protective supports the view that qualitative differences between the two resting populations, the naive and the memory population, may play a major role in their protective value against disease.

Longitudinal Analysis of the Acute Sendai Virus-Specific CD4+ T Cell Response and Memory

The development and persistence of Sendai virus-specific CD4+ T cell memory has been analyzed following respiratory infection of C57BL/6J mice by determining the prevalence of IL-2-producing Th cell precursors (Thp). Frequencies as high as 1:40 virus-specific CD4+ T cells were found in the regional lymph nodes and spleen during the acute phase of the host response and persisted at levels ≥1:500 for 2 to 3 mo. Thereafter, these CD4+ T cells tended to distribute more to the spleen than to the lymph nodes, a pattern that persisted for the life of the animals. From 3 to 12 mo after infection, virus-specific Thp were always detectable, although the numbers were diminished relative to those measured during the acute phase. Thereafter, however, in both contemporary and cumulative assays, there was a progressive increase in both the frequency and number of Thp. These increases were especially apparent for mice more than 2 years of age. This may reflect enrichment of the CD4+CD44high memory set due to the gradual diminution of the naive CD4+CD62LhighCD44low component. Analysis of DNA staining profiles for the CD4+ T cells showed high levels of cycling for the acute phase of the response, whereas the rate of T cell turnover measured for the CD4+CD44high population by bromodeoxyuridine incorporation indicated a pattern of stable, continuing proliferation throughout life. Virus-specific CD4+ T cell memory resulting from a single exposure to a readily eliminated RNA virus is thus maintained indefinitely in laboratory mice.

Stability of Virus-Specific CD4+ T Cell Frequencies from Acute Infection into Long Term Memory

Mice infected with viruses develop long-lasting high frequency memory CD8+ T cell pools, but much less is known about the CD4+ T cell response. FACS analysis revealed the modulation of several activation markers on CD4+ T cells during an acute infection with lymphocytic choriomeningitis virus (LCMV), consistent with an activated cell phenotype. Examination of virus-specific cytokine production using ELISPOT assays showed a significant increase in the number of IFN-γ-secreting cells in the spleen during an acute LCMV infection. CD8+ T cells made up the majority of the IFN-γ-producing cells, but analysis of the cell culture supernatants by ELISA showed that the CD4+ T cells produced more IFN-γ on a per cell basis. Using limiting dilution assays, we examined the CD4+ T cell precursor (Thp) frequency in C57BL/6 mice infected with LCMV. The virus-specific Thp frequency increased from <1/100,000 in uninfected mice to a peak of ∼1/600 in purified splenic CD4+ T cell populations by 10 days postinfection with LCMV. After the peak of the response, the Thp frequency decreased only about twofold per CD4+ T cell to ∼1/1200 and remained stable into long term memory. In contrast to the highly activated CD4+ T cells recovered during the acute LCMV infection, the memory CD4+ T cells were maintained at a lower activation state as judged by cell size and ability to secrete IFN-γ. Thus, like the CD8+ T cell frequencies, the CD4+ T cell frequencies remain elevated after the acute infection subsides and stay elevated throughout long term immunity.