First printed in R&D Systems' 2000 Catalog.
Immunity is the result of interplay between two "immune" systems: the innate immune system that initially encounters antigen and the adaptive immune system of T and B cells, which responds to information provided by the innate system. A hallmark of the innate system is the non-specificity of antigen identification. By utilizing a relatively small number of "pattern recognition receptors" that recognize highly conserved native molecular patterns on microbes, a large number of diverse organisms can be detected by a limited number of cell types.1-3 Cells that contribute to this system include phagocytes (neutrophils and monocytes), macrophages, dendritic cells and dendritic cell precursors, NK cells, gd T cells, and likely mast cells.1-8 Once antigen has been detected by an innate immune cell, this information is communicated to T and B cells of the adaptive immune system.1,3,4 Although T and B cells need to be instructed how to respond (i.e., what specific cytokines to secrete, what antibody isotype(s) to make), they demonstrate the remarkable facility of immunologic memory. The innate immune system provides signals for the activation of the adaptive immune system. It does so by providing signals related to the context and molecular nature of antigenic epitopes. The signals that are sent indicate whether antigen should be attacked and, if so, how? The adaptive immune system induces T cells to change from a naive phenotype to either an effector functional type or a memory phenotype. The Th1/Th2 phenotype reflects the outcome of naive T cell activation.1-3,7
|Figure 1. Schematic representation of cytokines influencing the development of antigen-activated naive CD4+ T cells into Th1 and Th2 cells.|
The abbreviations Th1 (T helper cell type 1) and Th2 (T helper cell type 2) refer to CD4+ g; Th2: IL-4 and IL-5), they can be considered Th1 or Th2 primary effector cells.21, 22 If they are "resting" but polarized (i.e., committed to a Th type), they could be considered Th1 or Th2 memory cells21,22 which, when reactivated, form Th1 or Th2 memory effector cells.22 Memory cells, at least in the CD8+ lineage, are a distinct stage in T cell differentiation and not a transition state in effector cell development.23 Whether memory cells derive directly from antigen-stimulated naive cells21 or result from a down-activation of functioning effector cells has yet to be determined.21,24
The Th phenotypes are characterized by the cytokines they produce. The first Th cell types reported were mouse Th1 and Th2 cells. Mouse Th1 cells were found to secrete IFN-gamma, while Th2 cells secreted IL-4.25 In the human, Th1 cells were also identified that secrete IFN-gamma,9,12 but their actual existence is controversial. Since cytokine production is usually measured in a heterogeneous population of cells, the Th0 cell may simply represent a mixture of Th1 and Th2 cells.17,19
In addition to cytokine production profiles, there are a number of cell surface markers proposed to differentiate Th1 vs. Th2 subtypes. For example, Th1 cells express both components of IL-12 receptor chains (beta 1 and alpha.32,33 Only Th2 cells appear to express a fully functional IL-1 receptor,34 and ST2L/T1, a newly discovered IL-1 RI-like molecule, is found on Th2 cells only.35 Chemokine receptors CXCR3 and CCR5 are characteristic of Th1 cells,36,37 while CXCR4, CCR3, CCR4, CCR7 and CCR8 are associated with Th2 cells.37,38 CD30, a member of the TNF superfamily, is associated with Th2 cells.15,39
A number of factors have been suggested to impact the development of Th1 and Th2 cells. All seem to be dependent, however, on a fundamental stepwise interaction between the antigen presenting cell (APC) and the naive/Thp cell. This interaction begins with the presentation of an antigen-MHC class II complex on the surface of an APC to the TCR/CD3/CD4 complex on naive T lymphocytes.40,41 This interplay activates the naive T cell, resulting in IL-2 receptor expression, IL-2 secretion, and CD40L upregulation. IL-2 interacts with IL-2R in an autocrine manner, while the appearance of CD40L allows the T cell to bind constitutively expressed CD40 on the surface of the APC. This interaction stimulates the APC to first express CD86/B7-2, and later CD80/B7-1. These molecules serve as membrane-bound ligands for T cell membrane CD28. The B7-CD28 interaction is a key connection, because CD28 ligation: 1) amplifies IL-2 secretion (and thus proliferation), 2) induces the appearance of the anti-apoptotic molecule Bcl-x (promoting survival), and 3) may contribute to future cytokine secretion.41-44
With ligation of CD28, the naive T cell may differentiate along more than one pathway, subject to a variety of inputs.16 One factor that may influence Th development is the MHC-TCR interaction itself. Very low and very high antigen doses have been suggested to promote a Th2 response, while moderate antigen levels predispose naive cells to become Th1 cells.45 Alternatively, when dose and affinity of antigen are considered concurrently, exact opposite results are reported. Low and high doses of high affinity antigens yield Th1 cells, while moderate doses of high affinity antigens yield Th2 cells.46 Antigen, at almost any dose, favors a Th0 phenotype and the key to subsequent differentiation is the level of available IL-2.47 Although CD4 is part of the TCR complex on naive T cells, it does not appear to be required for either Th1 or Th2 development.48,49
Co-stimulatory molecules have also been investigated for their effects on differentiation. Within the B7-CD28 system, CD86/B7-2 is associated with Th2 development9,50,51 while CD80/B7-1 delivers a neutral differentiation signal.52,53 These effects may be cytokine dependent.53 Other co-stimulatory molecules found on T cells include ICOS, a newly discovered CD28-like molecule that may contribute to Th2 development,54,55 and two TNF superfamily members, OX40 and 4-1BB, that may predispose to Th2 and Th1 development, respectively.55-57
The time of availability plus the relative ratio(s) of cytokines systematically drives naive T cells to one or more fundamental phenotypes.12,16 Aside from the issue of IL-2, whose role in differentiation may be restricted to select stages of Th2 development,58,59 three cytokines seem central to the initial stages of development of Th1 and Th2 cells. The first is IL-4, a 20 kDa monomer secreted by Th2 cells, mast cells, basophils, and eosinophils. The second is IFN-gamma, a 35 kDa noncovalent homodimer that is secreted by a variety of cells, including NK cells, Th1 cells, macrophages, and gd T cells. The third is IL-12, a 70 kDa heterodimer that is secreted by APCs, neutrophils, and keratinocytes.
IL-12 and IL-4 have been considered the pivotal cytokines in influencing antigen-activated naive CD4 + T cells to develop into Th1 and Th2 cells, respectively.9,12,60-62 Not all cytokines are equal in their effects, however, and this is in part due to variability of receptor expression. When T cells are in the Thp/naive state they are IL-4 R+,18,63 IL-12 R beta1-beta2-64 and IFN-gamma R alpha+ beta+.65 Once the Thp cell is antigen-activated via interaction with an APC (as described above), its existing IL-4 R is upregulated,63 both IL-12 R beta 1 and beta 2 appear,62,64 while IFN-gamma R alpha and beta expression is maintained.65 In this transitory, Th0-like state, all relevant receptors appear to be expressed. From this point, the quantities and timing of appearance of various cytokines are determinative. IL-12, a Th1 growth factor, is secreted almost immediately by APCs through their antigen presentation and B7 ligation.60,66,67 IL-12 binds to NK cells and Th0 cells, inducing rapid synthesis of IFN-gamma.16,68,69 This initial induction of IFN-gamma in a Th0 cell leads, first, to an apparent reinforcement of IL-12 R subunits, 17 and, second, to a downregulation of its own IFN-gamma R beta subunit.17,65 This, in theory, yields an IFN-gamma secreting cell (i.e., Th1 cell) that is now unresponsive to its own IFN-gamma, still responsive to IL-12, and potentially responsive to IL-4 via its IL-4R.18 The existence of the IL-4 R on fully differentiated Th1 cells must be emphasized, because this provides a mechanism for future functional modulation by IL-4.
IL-4 is considered dominant over IL-12. It upregulates expression of its own receptor, inhibits the secretion of IL-12 and downregulates the expression of the beta 2 subunit of the IL-12 receptor. It likely induces its own expression (in both naive and effector cells), and it is reported to induce a Th1 to Th2 switch, possibly through activation of its own receptor on Th1 cells.18,61,62,70,71 IL-12, in contrast, cannot block IL-4 production and cannot induce a Th2 switch to Th1. This may relate to the fact that Th2 cells are constantly making IL-4, and IL-4 downregulates the IL-12 R beta, any lag in IL-4 appearance may not be important as long as Th1 cells can later be potentially converted to Th2 cells. In any event, IL-4 is dominant, and will prevail if it reaches a critical level.66
The discussion above is an oversimplification of a very complex system. For example, TGF-beta in the presence of IL-4 and high IL-2 may drive naive T cells to a Th1 phenotype.74 The Th0 stage may be bypassed entirely if cytokines are immediately available.75 It is suggested that IL-12 can promote Th2 responses in effector cells.60 Memory Th1 cells may not be converted to Th2 cells, while Th2 memory cells may be converted to Th1 cells.76 In summary, the extent of T cell diversity is not fully understood.9
Th1 and Th2 cells have been associated with specific immune responses due to the cytokines they secrete. For pathogens that require internalization, the presence of Th1 cytokines (IFN-gamma and TNF-beta) is considered necessary. Conversely, for large extracellular parasites such as helminths, Th2-type cytokines (IL-4 and IL-5) have been considered most protective.13,78-80 In the case of Th1-type cytokines, IFN-gamma has a multitude of functions. It promotes phagocytosis and upregulates microbial killing. In particular, it induces IgG 2A (in mice) which is known to opsonize bacteria. On phagocytes, IFN-gamma promotes the expression of Fc gamma RI receptors, which are used for phagocytosis. It further upregulates the availability of NO, hydrogen peroxide, and superoxide in cells actively participating in phagocytosis. IFN-gamma provides all the tools necessary to eliminate most external microbes.61,79-81 To guarantee that monocytes/macrophages and T cells get to the site of infection, IFN-gamma works in concert with TNF-beta/LT-alpha to induce endothelial cell expression of adhesion molecules specific to monocytes and T cells, and promotes the expression of chemokines that specifically attract mononuclear cells (i.e., IP-10, MIG, RANTES, and MCP-1).81,82
For the classic Th2 cytokine, IL-4, its secretion triggers a number of events that parallel those of IFN-gamma. IL-4 promotes production of neutralizing antibodies (IgG) and the mast cell/eosinophil degranulating antibody known as IgE. 61,81 It also promotes upregulation of IgE receptors on mast cells, eosinophils and macrophages, and it induces membrane expression of macrophage MHC class II molecules and the IL-4 receptor.61 IL-4 and IFN-gamma often exist in an antagonistic relationship. IFN-gamma blocks IgE and IgG1 production, while IL-4 blocks IgG2A secretion. 81 Although Th2 cells have been associated with helminth infections, the roles that IL-4 and IL-5 play in protective immunity are unclear. IL-4 has a strong association with the clearance of intestinal worms. This effect, however, may be on non-immune cells.78 IL-5 likely activates eosinophils against parasites present in tissue. This could complemented by IgE, which would be directed against parasite antigens and may serve as an opsonizing factor that induces toxic granule secretion by eosinophils,13,78,82,83 and by IL-4 itself, which is known to induce the expression of endothelial adhesion molecules that draw eosinophils to the site of infection. 84