Memories that last forever: strategies for optimizing vaccine T-cell memory

Abstract

For acute self-limiting infections a vaccine is successful if it elicits memory at least as good as the natural experience; however, for persistent and chronic infections such as HIV, hepatitis C virus (HCV), human papillomavirus (HPV), and human herpes viruses, this paradigm is not applicable. At best, during persistent virus infection the person must be able to maintain the integrity of the immune system in equilibrium with controlling replicating virus. New vaccine strategies are required that elicit both potent high-avidity CD8(+) T-cell effector/memory and central memory responses that can clear the nidus of initial virus-infected cells at mucosal surfaces to prevent mucosal transmission or significantly curtail development of disease. The objective of an HIV-1 T-cell vaccine is to generate functional CD8(+) effector memory cells at mucosal portals of virus entry to prevent viral transmission. In addition, long-lived CD8(+) and CD4(+) central memory cells circulating through secondary lymphoid organs and resident in bone marrow, respectively, are needed to provide a concerted second wave of defense that can contain virus at mucosal surfaces and prevent systemic dissemination. Further understanding of factors which can influence long-lived effector and central memory cell differentiation will significantly contribute to development of effective T-cell vaccines. In this review we will focus on discussing mechanisms involved in T-cell memory and provide promising new approaches toward expanding current vaccine strategies to enhance antiviral memory.

Figure 1

Phenotypic markers associated with antigen-induced differentiation of naive CD8⁺ T cells. Within 48 hours after TCR activation, naive and memory CD8⁺ T cells up-regulate activation markers including IL-2R, Ki67, PCNA, and respond to signals delivered through JAK/STAT pathways to undergo changes in chemokine and homing receptors. CD62L is down-regulated and cleaved from the cell surface and IL-7Rα (CD127) expression is lost. During the proliferative phase a small subset of cells defined as memory precursor effector cells (MPECs) reacquire the IL-7Rα and have the potential to persist into long-term memory whereas the population that fails to up-regulate this receptor represent short-lived effector cells (SLECs).^,, The antigen-specific T-cell population that fails to up-regulate IL-7R expresses high levels of killer cell lectin-like receptor G1 (KLRG1). Although all antigen-activated CD8⁺ T cells are thought to express immediate effector function and proliferative capacity, transition to memory is dependent upon multiple sequential signals received by the T cell including the intensity and duration of TCR activation, CD4⁺-proficient help, costimulation, and cytokines that regulate survival. Transition to memory after natural infection is more dependent on IL-7, whereas T cells that receive weaker signals in the case of soluble protein antigens delivered with adjuvants are equally dependent on IL-7 and IL-15 for survival and transition to memory. Defining phenotypic and functional markers that characterize different transitional phases of CD8⁺ T cells induced by different viral vectors and other delivery systems will be instrumental in advancing strategies for effective T-cell vaccines for HIV-1, cancer, and other infectious diseases. CD45RA also known as leukocyte common antigen, highly glycosylated protein tyrosine phosphatase regulating TCR, BCR activation and found on naive/resting T cells; CD44, family of cell-surface glycoproteins involved in leukocyte attachment and rolling on endothelial cells and homing to peripheral lymphoid organs by binding mucosal addressin on high endothelial venules. Marker for antigen-experienced cells; CCR7, CC-chemokine G protein–coupled receptor guides cells from peripheral tissue into lymph nodes binding CCL19 (Mip3β) and CCL21(SLC) deposited on HEVs and reticular network; CD62L, L-selectin binds to CD34 and mediates lymphocyte homing through high endothelial venules of peripheral lymphoid tissue and inflamed tissue; CD127, also known as IL-7R; CD122, (IL-2β chain) pairs with γ common chain. Critical component of IL-2 and IL-15–mediated signaling; CD28, constitutive, low-affinity costimulatory signal induces T-cell activation, IL-2 production, and survival; CD27, TNFR superfamily member 7 binds to CD70. Costimulatory signal helps differentiate memory-type CD8⁺ T cells (CD27⁺) from effector-type CD8⁺ T cells (CD27⁻); CD43, leukosialin ligand-receptor complex involved in T-cell activation. Ligand for E-selectin and may regulate T-cell trafficking; CD95, TNF receptor superfamily member 6 also known as Fas. Cell-surface membrane receptor that activates apoptotic pathways when bound by Fas ligand (FasL, CD178); KLRG1, killer cell lectin-like receptor G1, senescence-associated inhibitory receptor binds E cadherin and inhibits AKT phosphorylation; Perforin, is indispensable for granule-mediated cell death by CD8⁺ CTL; Granzyme B, serine protease–inducing caspase-dependent apoptosis. Performs a key role in the cytotoxic activity mediated by CD8⁺ CTL (http://www.pathologyoutlines.com/cd100247.html).

Figure 2

Spatial-temporal signals determine the fate of activated CD8⁺ T cells. The balance between TLR-induced proinflammatory and apoptotic signals via STAT1 pathways in activating DCs and subsequent induction of negative feedback loops that initiate anti-inflammatory signals via the STAT3 axis define the narrow window of DC competency for polarizing CD4⁺ Th1 responses and cross-presentation of antigen to CD8⁺ T cells. TCR recognition of pMHC results in rapid down-regulation of CD62L, SIP1, and IL-7R, up-regulation of activation markers, and acquisition of effector function. During the next 3 to 4 days, Th1 CD4⁺ and CD8⁺ T cells proliferate in response to autocrine and paracrine IL-2, produce IFN-γ, and up-regulate CTLA-4. CTLA-4 ligation on DCs leads to indoleamine 2,3-dioxygenase (IDO) up-regulation, Foxo3-mediated inhibition of IL-6 production, and a shift from competency to induction of regulatory T cells, anergy, and cell death. This is the cue for activated CD4⁺ and CD8⁺ T cells to egress from secondary lymphoid organs. Foxo1 and KLF2 transcription factors, regulated by posttranscriptional modifications, coordinate renewed expression of the IL-7R, CD62L, CCR7, and S1P1, and down-regulate inflammatory chemokine receptors, respectively, on cells destined to become T central memory (Tcm) cells. In contrast, both IL-2 and IL-4 suppress Kruppel-like factor 2 (KLF2) expression and signals transmitted through IL-4R/ STAT6 up-regulate Eomes which induces expression of CXCR3 on CD8⁺ T cells. Survival after cytokine deprivation is dependent on costimulatory and other survival signals received before egress from lymphoid tissue. Limiting growth factors cause activated T cells to shutdown growth and proliferative programs sustained through TCR/CD28, and IL-2 and the P13K/AKT/mTOR pathway and to up-regulate autophagy pathways during transit into nonactivated lymphoid tissue and tissue niches. Here, T central memory (Tcm) and T effector memory (Tem) cell populations become dependent upon cytokine and tissue-specific interactions for maintenance and homeostasis. CD8⁺ T cells that receive CD4-proficient help in T cell–rich zones of draining lymph nodes may be more destined for long-term survival. The balance between costimulatory signals that up-regulate antiapoptotic factors, and negative costimulatory molecules up-regulated during the effector phase such as CTLA-4, BTLA-4, and PD1 (during chronic stimulation) that block effector function ultimately determine the population of cells that survive and transit into the memory pool. Expression of CD8⁺ effector function is regulated by 2 T box–binding transcription factors, Tbet and eomesodermin (Eomes). Although both Tbet and Eomes are essential inducers of CD8⁺ T-cell IFN-γ, perforin, granzyme B, and cytolytic capability, how these 2 transcription factors are coordinately regulated to control effector function and transition to long-term memory is unclear. During the inductive phase of the immune response, IL-12 and IFN-γ drive the differentiation and expansion of CD4⁺ Th1 and CD8⁺ T effector cells. This is reflected in the levels of T-bet expression and secreted osteopontin. During late-stage effector differentiation, inhibitor of DNA binding 2 (Id2), an antagonist of E protein transcription factors is up-regulated by CD8⁺ T cells and maintained in effector memory cells. E proteins are basic helix-loop-helix family of transcriptional activators and repressors, which bind specifically to DNA sequences containing the E-box consensus sequence. E protein homodimers regulate a complex array of genes during T-cell differentiation including expression of CD127 and CD27. Members of helix-loop-helix (HLH) protein family of Id (inhibitor of differentiation) dimerize with bHLH transcription factors and function as negative regulators of differentiation and promote progression of cells into S phase. E-Id2 heterodimer formation leads to diminished E-box–mediated gene expression such as Ctla4 and Bcl2l11 (BimEL) associated with reduced survival of T effector cells and relieves E protein repression of Serpinb9 coding for the serine protease inhibitor (Spi-6) thought to protect cytolytic effector cells from programmed cell death. Thus, Id-2 regulates the size of the Tem (CD62L^low, CD122^lowCD127^lowCD27^high) subset in addition to regulating the survival of effector CD8⁺ T cells. Understanding transcriptional programs that control proliferation, acquisition of effector function, and survival of CD4⁺ and CD8⁺ T cells during the immune response will be key to developing new vaccine strategies.