The goal of this body of research was to elucidate the mechanism by which CD4+ T cells provide help for CD8+ cytotoxic T lymphocyte (CTL) responses in different immunization types. The establishment of diseases, such as chronic infections and cancers, is attributed to severe loss of or dysfunctions of CD4+ T cells. Even in acute infections, CD4+ T cell deficiency leads to poor memory responses. While the role of CD4+ T cells is being increasingly appreciated in these diseases, the timing and nature of CD4+ T help and associated molecular mechanisms are not completely understood. Growing evidence suggests that, depending on the type of infections or immunizations, the requirements of CD4+ T cells can vary for optimal CD8+ CTL responses. In order to understand the modulatory effects of CD4+ T cells for optimal CD8+ CTL responses, two distinct immunization types were chosen. These include: 1) non-inflammatory dendritic cell (DC) immunization, which fails to provide inflammatory/danger signals; and 2) inflammatory adenovirus (AdV) immunization, which provides profound inflammatory/danger signals. This allowed us to study CD4+ T cell’s participation under different inflammatory conditions. The studies described in Chapters 2 and 3 of this thesis were performed to further understand the concept of how CD4+ T cells mediate optimal CD8+ CTL responses. This has been called the “new dynamic model of CD4+ T helper – antigen (Ag)-presenting cells (Th-APCs),” proposed in 2005 by our laboratory. The study described in Chapter 2 shows that Th-APCs participate not only in augmenting CTL-mediated immune responses, perhaps during early phase, but also in regulating cellular immunity, perhaps during a later phase. Through enhanced IL-2, CD80 and CD40L singnaling, and weaker peptideMHC I (pMHC) signaling, Th-APCs stimulated naïve CD8+ T cells to differentiate into effector CTLs, capable of developing into, central memory CTLs. Th-APC-stimulated CD4+ T cells behaved like Th cells in function, augmenting the overall magnitude of CTL responses. In contrast, Th-APCs were able to kill DCs and other Th-APCs, predominantly through perforin-mediated pathway. The experiments described in Chapter 3 revealed a novel co-operative role of cognate Th-CTL interactions, contrary to previously known immune-regulatory mechanisms among Th-Th or CTL-CTL interactions. In our experiments, Th cells, via CD40L, IL-2, and acquired pMHC-I signaling, enhanced CTL survival and transition into functional memory CTLs. Moreover, RT-PCR, flow cytometry and western blot analysis demonstrate that increased survival of Th cell-helped CTLs is matched with enhanced Akt1/NF-κB activation, down-regulation of FasL and TRAIL, and altered expression profiles with up-regulation of prosurvival (Bcl-2) and down-regulation of proapoptotic (NFATc1, Bcl-10, Casp-3, Casp-4, Casp-7) genes/ molecules. Finally, helped CTLs were also able to induce protection against highly metastasizing tumor challenge, explaining why memory CTLs generated under cognate Th1’s help show survival and recall advantages. The studies in Chapter 4 showed how the precursor frequency (PF) of CD8+ T cells impacts CD4+ T helper requirements for functional CTL responses. At endogenous PF, CD4+ T helper signals were necessary for both primary and memory CTL responses. At increased PF, CD4+ T help, and its CD40L but not IL-2 signal became dispensable for primary CTL responses. In contrast, memory CTL responses required CD4+ T cell signals, largely in the form of IL-2 and CD40L. Thus, these results could impact the development of novel immunotherapy against cancers, since their efficacy would be determined in part by CD4+ T help and CD8+ T cell PF. Finally, the study showed the importance of CD4+ T cells for multiple phases of AdV transgene product-specific CTL responses. These include: a) cognate CD4+ T cells enhanced CTL responses via IL-2 and CD40L signaling during primary, maintenance and memory phases; b) polyclonal CD4+ T environment enhanced the survival of AdV-specific CTL survival, partially explaining protracted CTL contraction phase; and c) during the recall phase, the CD4+ T environment, particularly memory CD4+ T cells, considerably enhanced not only helped, but also unhelped, memory CTL expansion. Thus, these results suggest the participation of both cognate and polyclonal CD4+ T cells for multiple phases of AdV-specific CTLs. Taken together, the current work delineated the critical roles of CD4+ T cells in different stages of CTL responses and in the development of anti-tumor immunity. The results presented here will significantly advance our current understanding of immunity to cancers, autoimmunity and chronic infections, since pathogenesis of these diseases is largely determined by CD4+ T helper functions. As most immunization procedures use the principle that is based on functions of memory cells, the knowledge gained from this work will also have a major impact on designing vaccines against intractable diseases, including cancers and chronic infections. Moreover, in advanced tumors, vaccines developed using this knowledge may act synergistically with other cancer treatments such as irradiation, chemotherapy and microsurgery, minimizing their side effects and prolonging the lives of patients.