Oxidation-Specific Epitopes (OSEs) Dominate the B Cell Response in Murine Polymicrobial Sepsis.

In murine abdominal sepsis by colon ascendens stent peritonitis (CASP), a strong increase in serum IgM and IgG antibodies was observed, which reached maximum values 14 days following sepsis induction. The specificity of this antibody response was studied in serum and at the single cell level using a broad panel of bacterial, sepsis-unrelated as well as self-antigens. Whereas an antibacterial IgM/IgG response was rarely observed, studies at the single-cell level revealed that IgM antibodies, in particular, were largely polyreactive. Interestingly, at least 16% of the IgM mAbs and 20% of the IgG mAbs derived from post-septic mice showed specificity for oxidation-specific epitopes (OSEs), which are known targets of the innate/adaptive immune response. This identifies those self-antigens as the main target of B cell responses in sepsis.

Antibody Production in Murine Polymicrobial Sepsis-Kinetics and Key Players.

Although antigen-specific priming of antibody responses is impaired during sepsis, there is nevertheless a strong increase in IgM and IgG serum concentrations. Using colon ascendens stent peritonitis (CASP), a mouse model of polymicrobial abdominal sepsis, we observed substantial increases in IgM as well as IgG of all subclasses, starting at day 3 and peaking 2 weeks after sepsis induction. The dominant source of antibody-secreting cells was by far the spleen, with a minor contribution of the mesenteric lymph nodes. Remarkably, sepsis induction in splenectomized mice did not change the dynamics of the serum IgM/IgG reaction, indicating that the marginal zone B cells, which almost exclusively reside in the spleen, are dispensable in such a setting. Hence, in systemic bacterial infection, the function of the spleen as dominant niche of antibody-producing cells can be compensated by extra-splenic B cell populations as well as other lymphoid organs. Depletion of CD4+ T cells did not affect the IgM response, while it impaired IgG generation of all subclasses with the exception of IgG3. Taken together, our data demonstrate that the robust class-switched antibody response in sepsis encompasses both T cell-dependent and -independent components.

Polymicrobial sepsis and non-specific immunization induce adaptive immunosuppression to a similar degree.

Sepsis is frequently complicated by a state of profound immunosuppression, in its extreme form known as immunoparalysis. We have studied the role of the adaptive immune system in the murine acute peritonitis model. To read out adaptive immunosuppression, we primed post-septic and control animals by immunization with the model antigen TNP-ovalbumin in alum, and measured the specific antibody-responses via ELISA and ELISpot assay as well as T-cell responses in a proliferation assay after restimulation. Specific antibody titers, antibody affinity and plasma cell counts in the bone marrow were reduced in post-septic animals. The antigen-induced splenic proliferation was also impaired. The adaptive immunosuppression was positively correlated with an overwhelming general antibody response to the septic insult. Remarkably, antigen "overload" by non-specific immunization induced a similar degree of adaptive immunosuppression in the absence of sepsis. In both settings, depletion of regulatory T cells before priming reversed some parameters of the immunosuppression. In conclusion, our data show that adaptive immunosuppression occurs independent of profound systemic inflammation and life-threatening illness.

Reduced Numbers and Impaired Function of Regulatory T Cells in Peripheral Blood of Ischemic Stroke Patients.

BACKGROUND AND PURPOSE: Regulatory T cells (Tregs) have been suggested to modulate stroke-induced immune responses. However, analyses of Tregs in patients and in experimental stroke have yielded contradictory findings. We performed the current study to assess the regulation and function of Tregs in peripheral blood of stroke patients. Age dependent expression of CD39 on Tregs was quantified in mice and men. METHODS: Total FoxP3(+) Tregs and CD39(+)FoxP3(+) Tregs were quantified by flow cytometry in controls and stroke patients on admission and on days 1, 3, 5, and 7 thereafter. Treg function was assessed by quantifying the inhibition of activation-induced expression of CD69 and CD154 on T effector cells (Teffs). RESULTS: Total Tregs accounted for 5.0% of CD4(+) T cells in controls and <2.8% in stroke patients on admission. They remained below control values until day 7. CD39(+) Tregs were most strongly reduced in stroke patients. On day 3 the Treg-mediated inhibition of CD154 upregulation on CD4(+) Teff was impaired in stroke patients. CD39 expression on Treg increased with age in peripheral blood of mice and men. CONCLUSION: We demonstrate a loss of active FoxP3(+)CD39(+) Tregs from stroke patient's peripheral blood. The suppressive Treg function of remaining Tregs is impaired after stroke.

Full activation of CD4+ T cells early during sepsis requires specific antigen.

During sepsis, CD4 T cells express activation markers within the first 24 h. In the present study, the mechanisms of T-cell activation and its consequences were addressed in an acute peritonitis model in mice. The response of CD4+ T cells to sepsis induction was compared between OTII mice, characterized by ovalbumin-specific T-cell receptor-transgenic T cells, and C57BL/6 controls (wild type [WT] mice). Because ovalbumin was absent during peritonitis, the OTII CD4+ T cells could not be activated by canonical antigen recognition. In both OTII and WT control mice, CD4+ T effector cells and CD4+ Foxp3+ regulatory T cells (Tregs) expressed the activation marker CD69 early after sepsis onset. However, full activation with upregulation of CD25 and proliferation took place only in the presence of the antigen. Besides this, the fraction of Tregs was lower in OTII than that in WT mice. Sepsis mortality was increased in OTII mice. Our data show that, in sepsis, partial activation of CD4+ T cells is induced by a T-cell receptor-independent pathway, whereas full stimulation and proliferation require a specific antigen. Antigen-dependent T-cell effector functions as well as Treg activity may contribute to sepsis survival.

Experimental sepsis impairs humoral memory in mice.

Patients with sepsis are often immune suppressed, and experimental mouse models of sepsis also display this feature. However, acute sepsis in mice is also characterized by a generalized B cell activation and plasma cell differentiation, resulting in a marked increase in serum antibody concentration. Its effects on humoral memory are not clearly defined. We measured the effects of experimental sepsis on long-term immunological memory for a defined antigen: we induced colon ascendens stent peritonitis (CASP) 8 weeks after 2 rounds of immunization with ovalbumin. Four weeks later, the antigen-specific bone marrow plasma cell count had doubled in immunized non-septic animals, but remained unchanged in immunized septic animals. Sepsis also caused a decrease in antigen-specific serum antibody concentration. We conclude that sepsis weakens humoral memory by impeding the antigen-specific plasma cell pool's development, which is not complete 8 weeks after secondary immunization.

Foxp3+ regulatory T cells are required for recovery from severe sepsis.

The role of regulatory T cells (Tregs) in bacterial sepsis remains controversial because antibody-mediated depletion experiments gave conflicting results. We employed DEREG mice (DEpletion of REGulatory T cells) and a caecal ligation and puncture model to elucidate the role of CD4(+)Foxp3(+) Tregs in sepsis. In DEREG mice natural Tregs can be visualized easily and selectively depleted by diphtheria toxin because the animals express the diphtheria toxin receptor and enhanced green fluorescent protein as a fusion protein under the control of the foxp3 locus. We confirmed rapid Treg-activation and an increased ratio of Tregs to Teffs in sepsis. Nevertheless, 24 h after sepsis induction, Treg-depleted and control mice showed equally strong inflammation, immune cell immigration into the peritoneum and bacterial dissemination. During the first 36 h of disease survival was not influenced by Treg-depletion. Later, however, only Treg-competent animals recovered from the insult. We conclude that the suppressive capacity of Tregs is not sufficient to control overwhelming inflammation and early mortality, but is a prerequisite for the recovery from severe sepsis.

Heparin-induced thrombocytopenia: further evidence for a unique immune response.

Cardiopulmonary bypass surgery (CPB) is associated with a high incidence of IgG Abs against platelet factor 4/heparin (PF4/H) complexes by day 6 after surgery. These Abs are associated with an immune-mediated adverse drug reaction, heparin-induced thrombocytopenia. Although the early onset of the anti-PF4/H IgG response is compatible with a secondary immune response, the rapid decline of Ab titers thereafter is not. To shed light on the origin of these Abs, in the present study, we prospectively compared the kinetics of these Abs with that of Abs against 2 recall Ags and to that of autoantibodies in 166 CPB patients over 4 months. Surgery induced strong inflammation, as shown by an increase in mean C-reactive protein levels. Consistent with previous studies, anti-PF4/H IgG optical density transiently increased between baseline and day 10 (P < .001; not associated with C-reactive protein levels), followed by a decrease over the next months. In contrast, concentrations of antidiphtheria toxin IgG and antitetanus toxin IgG increased constantly over the 4 months after surgery by 25%-30%. IgG autoantibodies did not change. Therefore, the transient kinetics of the anti-PF4/H IgG response resembled neither that of recall Abs nor that of IgG autoantibodies, but rather showed a unique profile.

Sepsis leads to a reduced antigen-specific primary antibody response.

Immunosuppression, impaired cytokine production and high susceptibility to secondary infections are characteristic for septic patients, and for mice after induction of polymicrobial septic peritonitis by sublethal cecal ligation and puncture (CLP). Here, we demonstrate that CLP markedly altered subsequent B-cell responses. Total IgG and IgM levels, as well as the memory B-cell response, were increased in septic mice, but antigen-specific primary antibody production was strongly impaired. We found that two days after CLP, CD11b(+) splenocytes were activated as demonstrated by the increased expression of activation markers, expression of arginase and production of NO by immature myeloid cells. The in vivo clearance of a bacterial infection was not impaired. DCs demonstrated reduced IL-12 production and altered antigen presentation, resulting in decreased proliferation but enhanced IFN-γ production by CD4(+) cells. CD4(+) T cells from mice immunized on day 2 after CLP showed reduced Th1 and Th2 cytokine production. In addition, there was an increase in Treg cells. Interestingly, levels of immature B cells decreased but levels of mature B cells increased two days after CLP. However, adoptive transfer of naïve CD4(+) T cells, naïve B cells, or naïve DCs did not rescue the antigen-specific antibody response.

Platelet factor 4 binds to bacteria, [corrected] inducing antibodies cross-reacting with the major antigen in heparin-induced thrombocytopenia.

A clinically important adverse drug reaction, heparin-induced thrombocytopenia (HIT), is induced by antibodies specific for complexes of the chemokine platelet factor 4 (PF4) and the polyanion heparin. Even heparin-naive patients can generate anti-PF4/heparin IgG as early as day 4 of heparin treatment, suggesting preimmunization by antigens mimicking PF4/heparin complexes. These antibodies probably result from bacterial infections, as (1) PF4 bound charge-dependently to various bacteria, (2) human heparin-induced anti-PF4/heparin antibodies cross-reacted with PF4-coated Staphylococcus aureus and Escherichia coli, and (3) mice developed anti-PF4/heparin antibodies during polymicrobial sepsis without heparin application. Thus, after binding to bacteria, the endogenous protein PF4 induces antibodies with specificity for PF4/polyanion complexes. These can target a large variety of PF4-coated bacteria and enhance bacterial phagocytosis in vitro. The same antigenic epitopes are expressed when pharmacologic heparin binds to platelets augmenting formation of PF4 complexes. Boosting of preformed B cells by PF4/heparin complexes could explain the early occurrence of IgG antibodies in HIT. We also found a continuous, rather than dichotomous, distribution of anti-PF4/heparin IgM and IgG serum concentrations in a cross-sectional population study (n = 4029), indicating frequent preimmunization to modified PF4. PF4 may have a role in bacterial defense, and HIT is probably a misdirected antibacterial host defense mechanism.

Tumor necrosis factor-related apoptosis-inducing ligand (TRAIL) improves the innate immune response and enhances survival in murine polymicrobial sepsis.

OBJECTIVE: To investigate the role of tumor necrosis factor (TNF)-related apoptosis-inducing ligand (TRAIL) in postoperative polymicrobial abdominal sepsis.Sepsis is the leading cause of death among critically ill surgical patients. TRAIL is commonly known as an apoptosis-inducing agent in cancer cells. It also plays an important role in the regulation of inflammatory reactions. The role of TRAIL in polymicrobial sepsis is still unclear. DESIGN: Experimental animal model. SETTING: University laboratory. SUBJECTS: C57BL/6 mice. INTERVENTIONS: Colon ascendens stent peritonitis (CASP) was induced in female mice. One hour, 24 hrs, and 48 hrs after induction of CASP, murine recombinant TRAIL was given intravenously. MEASUREMENTS AND MAIN RESULTS: This study demonstrates a protective effect of TRAIL in CASP, an experimental model of murine polymicrobial sepsis. Intravenous administration of recombinant TRAIL to mice after CASP induction led to highly significantly prolonged survival. The migration of effector cells into the peritoneal cavity was strongly enhanced. Consequently, TRAIL-treated mice eliminated bacteria significantly better from the peritoneal cavity, the source of infection. Systemic spread of gut bacteria was also reduced by several orders of magnitude. As a result of the reduced systemic spread of bacteria, the accumulation of neutrophils within the spleen and mesenteric lymph nodes was strongly decreased. CONCLUSION: TRAIL-treated mice are highly protected from abdominal sepsis. Because diagnosis and therapy are frequently delayed in human sepsis, it is remarkable that TRAIL is effective when given via a therapeutic approach. Therefore, this study suggests a therapeutic potential for TRAIL in human sepsis. This should be addressed in future trials.