Hemophagocytic lymphohistiocytosis: a rare diagnosis, an even rarer opportunity to appraise our understanding of the immune system
While the diagnostic criteria include either the presence of a known genetic disorder or five of a list of eight signs and symptoms,
In a sense, all data seem to point to an immune dysfunction that fits perfectly with what one expects. The genetic disorders associated with the familial forms of HLH are those that affect the cytotoxic machinery of NK and T lymphocytes, compromising the cytotoxic granule contents themselves or the molecules involved in granule exocytosis, so that in any case, the cytotoxic function of these cells is faulty. Furthermore, even in sporadic cases, the frequent precipitation of this syndrome by viral infections or its association with NK and T-cell lymphomas indicates that there, too, defective cell cytotoxicity may be the culprit.
Actually, after the role of perforin deficiency was described in familial cases of HLH,
However, a simple explanation—lack of or defective cell cytotoxicity, allowing an unchecked viral expansion—does not explain all observations. It is true that in the perforin-deficiency model, an LCMV infection triggers the HLH-like syndrome and is associated with a very high viral load. However, this insufficient clearance of viruses is not enough to cause the disease. First, in the Munc13-4 orthologue gene deficiency, although the animals have poor cell cytotoxicity and are especially susceptible to the murine cytomegalovirus infection (MCMV), this latter disease does not trigger the HLH-like disease; only the LCMV infection does.
Moreover, pointing to a possible explanation for the phenomenon, in the same perforin-deficient animal model, the use of blocking antibodies against IFN-gamma also protects the animals from the HLH-like disease.
Complementing the hypothesis, data from the perforin-deficient animal model show that, in these mice, the absence of the adaptor molecule, MyD88, protects the animals from the HLH-like disease.
With these experimental observations, it is possible to elaborate a more detailed pathophysiological view of the HLH. This disease would be the consequence of an immune dysfunction, where the immune system, confronted by certain IFN-gamma-inducing challenges, fails to control its secretion due to a defective machinery of cytotoxic cells, namely NK and CD8+ T cells. The excessive IFN-gamma secretion, through a MyD88-dependent pathway, would cause macrophages to go rogue and further disrupt an already shaken physiologic balance in the host, prompting the HLH.
However well this proposed model explains the experimental data, one has to ask, does it fit the actual human disease? Initially, at the origin of the model, there is a good correlation between the genetic defects of mice and those found in patients with familial HLH. Furthermore, the explanations for the triggering of the disease in mice and humans coincide. But, when the model gets down to identifying the specific cytokine responsible for the generation of the disease, the model starts to fail: in humans, an elevated serum IFN-gamma, although sometimes found, is not typical of HLH.
Then again, one would be missing a real opportunity: to check and even challenge our understanding of the immune system. The immune system is very complex; it was selected to deal specifically with molecularly unknown challenges—the antigens—and it performs its function with exceptional efficacy. Our knowledge of the circuits that control this system is growing rapidly and is becoming so detailed that the whole picture starts to escape from many. In this context, the complexity of the circuits and their interaction requires the construction of (animal) models, where genes can be manipulated at will and hypotheses can be tested with precision. However, diving too quickly into the models may obscure the initial reason for their construction—the understanding of the immune system and how it interacts internally and with its many challenges in nature—and, for the physician, it may also unravel strategies to interfere when the immune system fails and disease ensues due to that failure. Then, reminding us that models are models and diseases are diseases, a syndrome like HLH is invaluable—as are autopsies and case reports—for in these instances, models are often insufficient and our relative ignorance shows up, driving us to challenge the models and deepen our understanding.
Specifically here, it is not the relative role of one or other cytokine that should cause the impact, but rather the role we ascribe to CD8+ T cells. These cells are identified as antigen-specific, HLA-restricted, cytotoxic cells that perform an essential role in viral infections and in tumor immunity. Evidence from this function comes from both in vitro observation that these cells are, indeed, able to kill specific targets—and this was central to unravel the role of the major histocompatibility antigens in restricting T-cell recognition of antigens,
However, the role of T-cell cytotoxicity in the control of virus infections in humans is not so clear-cut. Since children with immunodeficiencies characterized by the absence of CD8+ T cells are not more susceptible to viral infections than healthy children,
But the question remains: how would a cytotoxic defect prompt a deregulated cytokine secretion? We have seen that the viral load, per se, a simple and direct possibility, does not solve this issue; hence, the answer must be elsewhere. This brings to light another possible role of CD8+ T cells: immunoregulation. In the 1970s and early 1980s, T cells were classified as either helper T cells or suppressor/cytotoxic T cells. The latter population included cells clearly cytotoxic—those that are identified today as CD8+ T cells—and others that were indistinguishable from the cytotoxic cells by the surface markers then available, but whose function seemed to be the specific suppression of immune responses. This putative T-cell population did not resist deeper investigations, and later on, many immune regulatory circuits were described, including the “opposing” T helper subsets and the regulatory T cells, which could explain much of the earlier observations of specific immunosupression. Nevertheless, a “revival” of the regulatory role for the “cytotoxic” T cells seems to offer a solution in the case of HLH.
Thus, the pathophysiology of HLH could be described as an initial challenge to the immune system that drives the activation of CD8+ T cells. Due to a genetic defect (in the familial cases), or to local and/or transient conditions (in sporadic cases), these cells fail to perform an essential (and relatively ignored) role: the control of their own activation. With uncontrolled stimulation, CD8+ T cells cause other cells down the pathway of the immune response, like macrophages, to become further active, to secrete other cytokines, and, thus, trigger the disease. Therefore, in the end, it could seem that the pathophysiology of HLH is solved. Yet, this is not true. It could be enough to remember that it remains to be determined how the cytotoxic machinery of cytotoxic T cells affects immune activation and if it does truly occur in patients with HLH. But the uncertainties go further. The main characteristics of HLH are in the name of the syndrome itself: hemophagocytosis, lymphocytosis, and histiocytosis of tissues. Though a possible explanation for hemophagocytosis can be found on the action of cytokines (IFN-gamma in the model), the tissue infiltration by immune cells is not clearly explained. What drives their movement towards tissues? What keeps them active therein? And if we keep looking at case reports and autopsies of patients that presented HLH, new questions and new challenges will appear—and, hopefully, will drive our investigations towards a more comprehensive view of its pathophysiology and a more effective way to diagnose and treat it.