Cytokines are small proteins, secreted by many cells, including but not limited to immune cells such as lymphocytes and macrophages, which affect other cells that are involved in immunoregulation and inflammation. Cytokines are ubiquitous in the maintenance of innate and adaptive immunity and in response to foreign antigens and inflammatory stimuli. The complicated biology, mechanisms of action, and interaction with other cytokines and effector cells is the subject of several comprehensive reviews [[1], [2], [3], [4], [5], [6]].

Broad categories of cytokines include interleukins, interferons, tumor necrosis factor, chemokines and transforming growth factor-beta (Table 1). Colony stimulating (hematopoietic growth) factors are also often included in discussion about cytokines, given their cellular origin and pleotropic effects on the maturation and proliferation of other hemopoietic cells. Within each category, molecules with structural similarity are part of cytokine families. Cytokines with similar receptor binding and signal transduction pathways comprise cytokine superfamilies. For example, the interleukin (IL)-1 superfamily includes IL-1 alpha, IL-1 B, IL-1 delta, IL-18, IL-33, IL-36, IL-37 and IL-38 (7).

Cytokines don't exist in silos, however, and are part of a complex, and less than fully understood interplay of effector cells and regulatory and counter regulatory cytokines as well as other signaling proteins. Proinflammatory (e.g. IL-1, interferon-gamma and tumor necrosis factor-alpha) and anti-inflammatory (e.g. IL-4, !L-10 and TGF-beta) cytokines often work in concert with each other with the clinical consequences determined by the preponderance of cytokine type and the clinical substrate (Fig. 1). Multiple proinflammatory cytokines, including Interleukin-6, for example, have been implicated in cytokine release syndrome (CRS) caused by immune effector cell (IEC) therapy [8,9], whereas IL-10 has been shown to be instrumental in the development of organ transplant tolerance [10–12[. Cytokines can also have dual proinflammatory and anti-inflammatory properties. IL-6 both attracts and activates inflammatory leukocytes (macrophages and others) and is a major contributor to the acute phase reaction, but may also modulate alternative macrophage phenotypes, downregulating, for example, lipopolysaccharide induced inflammation [13,14].

Another classification which parallels their proinflammatory vs anti-inflammatory properties characterizes cytokines as either type1 (e.g., TNF-alpha and interferon-gamma) secreted by T helper 1 (Th1) cells that enhance cellular responses, and type 2 (e.g., IL4, IL-10 and transforming growth factor-beta) secreted by T helper 2 (Th2) cells that enhance antibody production [15,16]. Type 1 cytokines are instrumental in the response to intracellular pathogens and in promoting inflammation [15]. Type 2 cytokines are believed to be important in the response to certain pathogens such as parasites and helminths, in the pathobiology of immune mediated diseases such as asthma and in immunoregulatory functions such as transplant tolerance [16]. IL-10 signaling and enhancement of T regulatory cell development and function have been shown to be especially important regarding transplant tolerance [[10], [11], [12]].

In addition to the Th1/Th2 cell axes, Th17 cells are another CD4+ effector cell subset which produce the proinflammatory cytokines IL-17 and IL-22, and are involved in the response to pathogens, particularly of extracellular origin [[17], [18], [19]]. The dynamic balance between the Th1/Th2 subsets and associated cytokines and Th17 cells and cytokines is believed to be critical to immune homeostasis. Imbalances in that dynamic equilibrium, especially with polarization toward the Th17 subset, are believed to be important in the pathogenesis of autoimmune diseases [20].

Cytokines can also be considered as endogenous (i.e. secreted in response to an immune response or inflammation) or exogenous (cytokines such as granulocyte-colony stimulating factor and IL-2, in which the endogenous cytokine previously was identified and later manufactured as a structurally similar or identical protein using recombinant technology for therapeutic use). The identification of cytokines has had important therapeutic implications, as monoclonal antibodies and targeted small molecules directed against selected cytokines have important therapeutic benefit. For example, the monoclonal antibody tocilizumab targets the IL-6 receptor for the serious condition that may accompany CAR-T cell therapy, immune effector cell (IEC)-induced CRS [7,8], or the soluble TNF receptor etanercept that is employed as an immunosuppressant in treating rheumatoid arthritis [21,22]. The limited activity of certain monoclonal antibodies against specific cytokines, for example, anti-tumor necrosis-alpha antibody therapy for treatment of refractory acute graft versus host disease (GvHD) speaks to the limited role of individual cytokines in the milieu of other cytokines and immune effector cells in the pathobiology of the disease [23].

Cytokine profiles may be characteristic of certain disease states dominated by proinflammatory states, such as sepsis, acute and chronic GvHD and engraftment syndrome (ES) after hematopoietic cell transplantation (HCT), hemophagocytic lymphohistiocytosis (HLH), and autoimmune and autoinflammatory disorders [[24], [25], [26], [27], [28]]. In certain situations, specific cytokine profiles may differ between the diseases, suggesting different pathobiological mechanisms, and different avenues for targeted therapy. One study, for example, showed significantly higher serum IL-1- beta and IL-2 concentrations in pediatric patients with ES when compared with patients who developed acute GvHD associated with higher levels of IL-5, IL-7, IL-18, IL-10, IFN-gamma and TNF-alpha [29].

The onset of cytokine secretion varies according to the expected timing of complications after HCT with early CRS common after transplantation with the use of posttransplant cyclophosphamide (PTCy) for GvHD prophylaxis, particularly in the setting with use of HLA-mismatched donors. The later onset of cytokine secretion associated with acute and chronic GvHD (with chronic GvHD occurring typically 100 days after transplant but with considerable variability including the potential for an overlap syndrome) reflects different pathways of immune-mediated pathophysiology and different cytokine profiles [3,[24], [25], [26], [27],[30], [31], [32]].

Cytokines are also known to contribute to immunoregulatory responses in solid organ transplantation. Whereas type 1 cytokines have been historically associated with acute cellular rejection, type 2 cytokines have been implicated in antibody-mediated rejection [33,34]. Durable transplant tolerance, defined as the absence of a destructive immune response following organ transplantation in the absence of systemic immunosuppression, is considered the “Holy Grail” of human transplantation [35]. Tolerance has been achieved through hematopoietic chimerism following combined bone marrow and organ transplantation in preclinical small and large animal models as well as clinically [[36], [37], [38], [39]]. Combined bone marrow and kidney transplantation has been performed for patients with hematologic malignancy and end stage renal disease and for patients without an underlying malignancy. In the former circumstance, using a conventional reduced-intensity conditioning regimen and posttransplant cyclophosphamide for GvHD prevention, an early CRS is common prior to the PTCy therapy and can be managed with the anti-IL-6 drug tocilizumab (unpublished observations). For patients without an underlying malignancy, tolerance can be achieved through transient mixed hematopoietic chimerism [39].

Attempts to compare cytokine profiles between disease states have been hindered by the variable number of cytokines that were measured and the timing of the measurements in relation to the disease manifestations. In addition, technical aspects of measurement have included the variability of the assays that were used (plasma or serum cytokine assays vs mRMA measurements) and the conditions in which the samples were procured, as cytokines can degrade quickly in vitro, particularly in serum.