The study population comprised 40 subjects, including 8 females and 32 males, with mean ages of 62.6 ± 9.7 years and 57.9 ± 15.5 years, respectively. All the patients were infected with SARS-CoV-2. Fibrin thrombi were identified in 33 out of 40 patients (82.5%), and bacterial co-infections were detected in 33/40 (70%) of the cases (Table 1). The most frequently isolated microorganisms included Acinetobacter baumannii, Klebsiella pneumoniae, Pseudomonas aeruginosa, Staphylococcus epidermidis and Stenotrophomonas maltophilia.

The most common comorbidities in the population were systemic arterial hypertension, diabetes mellitus, acute kidney injury, and obesity (Table 1).

In light of the study's primary objective, which was to examine the structural pattern of fibrin in thrombi, patients with thrombi were classified into two groups, designated as the "Star-shaped pattern" and the "Reticular pattern" (Table 2), based on the observed fibrin structure in their thrombi. The analysis of the PaO₂/FiO₂ ratio and oxygen saturation (SatO₂) revealed no significant differences between the two groups (Table 2). Coagulation parameters, including prothrombin time, activated partial thromboplastin time, thrombin time, and D-dimer levels, were elevated above their respective reference values. However, no statistically significant differences were observed between the groups (Table 2). However, analysis of inflammatory indicator parameters indicated that the group of patients with thrombi exhibiting a star-shaped fibrin structure exhibited higher levels of C-reactive protein and neutrophil-to-lymphocyte ratio than the group of patients with a reticular fibrin pattern. Nevertheless, only CRP concentration exhibited a statistically significant difference (Table 2).

The lung tissue showed a patchy pattern, including areas with minimal and severe damage, separated by the interlobular septum. In areas exhibiting severe damage, interstitial thickening, edema, and inflammatory infiltrates, comprising neutrophils and lymphocytes, were observed. In contrast, regions exhibiting minimal damage displayed a paucity of the previously listed histopathological findings (Figure 1A). Hyaline membranes, pneumocyte detachment, intra-alveolar fibrin deposition, and alveolar macrophages were observed in the alveolar lumen in both areas regardless of the severity of the injury (Figure 1A and Figure 2A). The presence of thrombi was documented in both the arterioles (29/33) and pulmonary capillaries (19/33). In general, the pulmonary thrombi showed a characteristic reticular fibrin pattern (Figure 2C-D). However, in addition to this fibrin pattern, in 4/33 (12%) cases, the fibrin scaffold of the thrombus was observed to have a different arrangement than that of the classic reticular structure. Since we did not find any references describing this pattern, we call it "star-shaped fibrin pattern." Thrombi with star-shaped fibrin structures were observed in the pulmonary arterioles and capillaries (Figure 2E-F), predominantly in areas of severe lung injury (Figure 1C-E). Intra-alveolar fibrin deposits were observed in 24 of the 33 cases, but no star-shaped structures were observed, regardless of the severity of lung injury (Figure 2A-B). PTAH staining of the star-shaped structures demonstrated that they were composed of fibrin (Figure 2E-F and Figure 3A). Similarly, immunohistochemical staining with CD61 demonstrated the presence of platelet clumps (Figure 3B), which expressed the procoagulant proteins VWF (Figure. 3C) and Factor VIII (Figure 3D). Erythrocytes were not observed in fibrin structures.

Histopathological examination of renal tissue revealed tubular degenerative changes and glomerular damage. Fibrin thrombi were identified in 12/16 (75%) analyzed cases. These thrombi were predominantly located in peritubular (10/12) and glomerular (8/12) capillaries. The fibrin structure observed in renal thrombi was mostly fibrillary. However, in 4/12 (33%) cases, star-shaped fibrin structures were found in the glomerular and peritubular capillaries (Figure 3E-F). Collectively, thrombi with a star-shaped fibrin structure were observed in 7/40 autopsies (27.5%), with four cases located in the lungs and four in the kidneys. Of these, only one patient had this fibrin conformation in both organs.

The star-shaped fibrin structure is a conglomerate of discrete fibrin units, each consisting of a spheroid core, with fine fibrin needles radiating from it. The core had an irregular size with an average diameter of 4 µm. The overall size of the structure ranged from 12 to 15 µm. Thrombi with this fibrin pattern were observed more frequently in blood vessels located in areas of severe lung damage (1.04 ± 0.7 vessels/mm^²) compared to areas of minimal damage (0.33 ± 0.3 vessels/mm^²) (p = 0.012). In areas of minimal damage, this fibrin pattern was observed only in paraseptal pulmonary capillaries adjacent to areas of severe damage.

SARS-CoV-2 infection has been associated with several health complications, including arterial and venous thromboembolism, inflammation, hypoxia, immobilization, and diffuse intravascular coagulation. Our histopathological findings align with those of other autopsy series of COVID-19 patients, showing DAD, microthrombi in pulmonary arterioles and capillaries, and fibrin deposition in the alveolar spaces. Tubular degenerative changes and glomerular damage in our kidney samples were also consistent with findings from other studies on deceased COVID-19 patients. Nevertheless, to the best of our knowledge, this study is the first to document the structural alterations in the fibrin network that may be associated with SARS-CoV-2 infection. The only prior study on this phenomenon was conducted by Juhlin and Shelley. In 1977, they reported the in vitro formation of fibrin asteroid bodies, which have an amorphous central region, likely composed of platelet aggregates surrounded by fine, radiating needle-like crystals. This was achieved by incubating blood samples from patients with psoriasis or vasculitis with bacterial extracts or gram-negative bacterial endotoxins. It is important to highlight three key differences between the structures observed by Juhlin and those observed in the present study.

First, Juhlin's observations were made in vitro, whereas our observations were made in vivo.

Second, PTAH staining revealed that the nuclei were composed of fibrin rather than platelets. This finding was confirmed through immunohistochemical analysis, as the nuclei showed no reaction to CD61, VWF, or Factor VIII (Figure 3 A-D).

Third, the structures described by Juhlin range in size from 50 µm to 200 µm, whereas those described in this article range from 12 µm to 15 µm.

The results of Juhlin's study suggest that the structure of fibrin can be altered by bacterial products to adopt an asteroid shape. In light of these findings, we sought to investigate the potential correlation between bacterial co-infection and the formation of star-shaped fibrin structures in COVID-19 patients. Nevertheless, Fisher's exact test did not reveal a correlation between the fibrin pattern and the incidence of co-infection (p = 0.397).

An alternative approach to explain the formation of star-shaped fibrin bodies described in our study was to explore the potential relationship between coagulation and systemic inflammation parameters with the histopathological analysis. Although coagulation parameters were statistically similar in both groups, the analysis of systemic inflammation parameters CRP and NLR showed a tendency to be higher in the group with star-shaped fibrin bodies compared to those in the group with a reticular fibrin pattern (Table 2). Although only the CRP levels reached statistical significance, this trend in inflammation parameters strongly supported the histopathological findings, which revealed the presence of star-shaped fibrin thrombi predominantly in vessels located in areas of lung tissue with marked inflammation. Therefore, it is possible to hypothesize that this pattern may be related to thrombi inflammation induced by the host's local immune response to SARS-CoV-2 infection.